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Nobadays
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# Posted: 7 Jan 2025 08:29am
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My campervan has an Onan 2800 watt generator under it. I'm trying to determine how many amps the starter draws. Anyone?
The lithium battery thread got me questioning if I can use my new LiFePo4 house batteries to start the generator... it was originally hooked to the House FLA batteries. A review of the battery manual and it states, Do Not Use as a Starting Battery. I have searched the internet to no avail and the Onan manual doesn't provide that information.
I plan to run a wire to the vehicle battery for the generator starter and want to fuse that line. I don't know what size of fuse. The original line to the FLA had no fuse.
What I do know:
1. The wire to the generator starter is 8 AWG which is rated at 55 amps maximum.
2. Basically a one cylinder lawnmower engine.
3. I did start it once with the lifepo4 battery that has a 100a/1s BMS. So definitely less than 100amps and I cranked longer than 1 second so likely way less than 100 amps.
Unfortunately I don't have a way to check the amp draw... and no spare FLA here in AZ to hook to it to test anyway - half a dozen at the cabin...
Any idea? Thanks!
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gcrank1
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# Posted: 7 Jan 2025 09:00am
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Fwiw, You use a wire gauge with capacity above the load and you fuse to the wire gauge (ie, fuse protects the wire not the device).
Ive heard of heavy duty load testing meters, never seen or been around anybody with one afaik. You certainly don't want to try any common 'bench testing meter'!
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Brettny
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# Posted: 7 Jan 2025 10:30am
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With that small of a engine/starter you will be fine. Remember how small lawnmower batteries are?
You also have a BMS to protect it.
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paulz
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# Posted: 7 Jan 2025 11:08am - Edited by: paulz
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You mentioned in the Lfp thread that you had already turned it over with the litime? I also posted the cold cranking amps listed for my larger Onan in that thread. I haven’t tried mine yet but should be fine as Brett said. My Honda v twin cart is running one and I have started my small truck. I’m using a $100 Lfp so killing one doesn’t mean too much.
Couple of things concern me more: One, even though they work am I degrading the Lfp by loading it like that. And two, the Onan, at least mine, charges the battery while running. Risking damaging the alternator or battery when running?
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gcrank1
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# Posted: 7 Jan 2025 01:51pm
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Fwiw, just spit-ballin here....Im thinking that a small engine starter powered by an LFP cant be any worse on the battery than running a high load in the cabin off a big inverter; say a well pump, etc. with a hard start up load.
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ICC
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# Posted: 7 Jan 2025 02:45pm - Edited by: ICC
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Quoting: gcrank1 Fwiw, You use a wire gauge with capacity above the load and you fuse to the wire gauge
True for 'normal' power circuits. Engine starters all have the ability to draw more amps than what the ampacity charts indicate for the rating. They only draw the larger numbers for short intervals though, whereas the ampacity charts are for continuous ratings.
If I was fusing a cable from a battery to a starter motor I would use as large a fuse as needed to cover the starter draw, not the wire rating.
I would probably not fuse the cable to the generator starter. But I would route the power through a relay so the cable would only have power when cranking. I would be very careful selecting the wire path, perhaps armoring it in some places. If I did fuse it I would use a very fast blow fuse, such as a Type T. And carry at least one spare or some HD aluminum foil. 
I think the small engine starter might be completely safe to crank off an LFP battery. My Tacoma V6 draws 150 to 200 amps, IIRC. The Dodge Cummins hits 400 amps but that is a beast compared to the Onam single gas burner.
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ICC
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# Posted: 7 Jan 2025 02:56pm - Edited by: ICC
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Quoting: paulz the Onan, at least mine, charges the battery while running. Risking damaging the alternator or battery when running?
Different generators are going to have different systems.
My RVbus generator used the house batteries to start. The house batteries were only charged through the separate house charging system which took AC current from the AC generator output (or shore power).
In other words, the generator did not charge the starting battery via a self contained DC charging system like a car or motorcycle..
If a generator has a self contained battery and DV charger then there could be an issue with charging an LFP. As mentioned somewhere here, when I wanted an LFP for the firt bike the battery guys had a list of bikes that were 100% compatible. So that could be a problem with using an LFP battery with some generator packages. Ask the maker?
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paulz
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# Posted: 7 Jan 2025 03:39pm - Edited by: paulz
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There is a thread on diy solar about running LFPs on Onans. I don’t read that forum, just saw it on a google search. I have a pretty good fla on it right now, not in any hurry to switch over. Maybe Nobs or someone who uses that forum could ask.
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Nobadays
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# Posted: 7 Jan 2025 04:16pm
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Quoting: ICC The house batteries were only charged through the separate house charging system which took AC current from the AC generator output (or shore power).
Yes, this is how mine works.
Quoting: gcrank1 Im thinking that a small engine starter powered by an LFP cant be any worse on the battery than running a high load in the cabin off a big inverter; say a well pump, etc. with a hard start up load.
Probably. The manufacturer says not to but I know it works and obviously drawing way less than the 100 amp max of the BMS. With the 8 AWG Wire rated at 55A I'm guessing the starter motor draw is even less than that. However as ICC points out, the rating on wire is usually for a constant load not an intermittent one. But... that said I'm pretty sure the BMS would have stopped the output if it had exceeded 100A.
We rarely use the generator other than to exercise it once a month. I bet we haven't used it as a power source for the A/C or microwave more than 10 times in the 12 years we've owned the van. We use a 600w inverter to power one dedicated receptacle for our coffee grinder in the mornings. We really have no other need for AC current.
I just wish I knew how much it draws... I think it is likely I'll still run a line to the Chassis Battery.... more searching I guess.
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Nobadays
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# Posted: 7 Jan 2025 04:30pm
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I did find this:
A Predator 3500 generator starter motor typically draws around 29 amps when starting, based on its rated power output and the standard voltage of 12 volts; however, this can vary slightly depending on the specific model and battery conditions.
Guessing my Onan 2800 isn't much different.
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gcrank1
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# Posted: 7 Jan 2025 05:40pm - Edited by: gcrank1
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I just remembered that my 5000w gen (in storage so Ive forgotten it) had a 20ah sized bat (Chinese LA, died years ago) and pretty small 'cables to grd and starter solenoid. So small I was surprised. There were times it didn't start well and got cranked quite a bit and they never heated up.
Since then Ive jumpered an old std garden tractor LA (that wont turn the 7.5hp Kohler mower tractor engine enough to start it) that spins the v-twin gen engine fine. Once running the gen runs without a battery, it only needs it to start.
I'll likely use my LFP when I find that old LA has died over the winter.
Thinking about the heavier solar panel wiring that is 8ga I think that some leftover of that will work fine.
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Nobadays
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# Posted: 8 Jan 2025 09:22am
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I just read through the lifepo4 thread where I had commented on my LiTime/ LFP conversion in my van. Apparently fresher in my mind back then!
So my battery has a 500a/1s limit on discharge not the 100a I had stated here. I have no idea where I got the "<25a" draw to start the generator when I posted there. Hmmmm.
Well, still searching for the amperage draw for the Onan 2800. That said im guessing under 50a and likely in ~30a - ~40a range. That would still be a pretty big draw.
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gcrank1
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# Posted: 8 Jan 2025 10:10am
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Big draw at initial spin, especially when the engine is coming up on compression. Once the spin is going the draw load drops way off quickly and if our world is good the engine starts right up.
Again, in my mind, not unlike a big well pump at first and maybe better because our gen engine should start and the starter motor stop. That well pump keep pumping and if it is night and running off the bat bank and inverter there is no solar supplement.
Also consider that if that Onan had a pull start you'd be able to pull start it, it isn't like a 350 Chevy.
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travellerw
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# Posted: 8 Jan 2025 01:07pm
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Quoting: Nobadays Well, still searching for the amperage draw for the Onan 2800. That said im guessing under 50a and likely in ~30a - ~40a range. That would still be a pretty big draw.
The problem with a starter is that it can draw WAY WAY more initially. The biggest cause is that it takes much more energy to get a stationary object moving than keeping it moving. I would not be surprised if the initial inrush on that starter was 70-80 amps for the first second. The only real way to know is to use a quality DC clamp meter with a high trigger.
The second problem with a starter is the collapsing field when its de-energized. The magnetic field in the starter collapses generating a voltage spike that is fed back through the system. That voltage spike can absolutely kill a BMS (BMS contains mosfets that are sensitive to spikes). Old FLA batts don't care as the spike just dissipates in the giant hunks of lead. BMS designed for starting motors will have a "clamping" circuit to protect them from that voltage spike.
Personally, if my battery said not to use as a starter battery, I wouldn't. The BMS was not designed for that purpose and that is why they put that warning on it.
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gcrank1
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# Posted: 8 Jan 2025 02:29pm
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All that is thought provoking.....
And underscores the adage that 'for LFP forget everything you know about LA'.
Maybe I'll just buy a cheap? LA 30-35ah LA lawn tractor battery to use for jumping small engine stuff. The posts on those tend to be the upright rectangle of lead with a bolt hole. Ive found that a short black rubber bungie slips in the holes nicely for a carry handle. One of those weighs about what my 100ah LFP does, not to bad to move around.
Bet you could mount one of those up in the gen bay rather than running cable from the engine bay.
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paulz
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# Posted: 8 Jan 2025 03:38pm
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Well I’ll be dipped. All this time I thought my Onan charged the starting battery. Just checked, 12.4 sitting, 12.4 running. This was a new unit, still on the pallet, given to me by the local disaster recovery chief when they moved. I have used it quite a few times to charge my bank, always assumed it was charging the battery. Guess not.
Anyway while I was playing around I pulled the battery and hooked up a 100ah Lfp. Not even fully charged, read 13.15v. Started it right up like nobodies business, same as it does for my Honda cart.
I know your not supposed too..
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ICC
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# Posted: 8 Jan 2025 04:00pm
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Thanks travellerw, That is good to know.
A good surge suppressor such as one of the Midnite Solar MNSPD that are used for solar lightning protection would probably work, but that is about $100 and only a guess.
Quoting: gcrank1 Bet you could mount one of those up in the gen bay rather than running cable from the engine bay.
A generator that is designed for use in an RV usually does not have a 12 volt charging system like a car, motorcycle or portable electric start generator. The few I have seen have all used the house batteries as a cranking battery. (House batteries being those that power the RV when camped). The house batteries are charged by the generator or shore power. Solar if you have it.
So with LFP house batteries having a separate battery to crank the generator, one would need a method to charge that battery. Or skip the extra battery and run a cable to the main engine battery.
I used a cable from the engine cranking batteries with a relay.
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paulz
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# Posted: 8 Jan 2025 07:47pm
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Yeah I’m thinking about cabling mine to the battery bank, now that I know it needs charging. Problem might be if the bank dies, no genny startup.
Other thing, if you look at my Onan tag, it says fuel - gasoline. When I got it I’m sure it was set up for LP. That’s all I’ve ever run it on.
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travellerw
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# Posted: 8 Jan 2025 08:26pm
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Quoting: ICC A good surge suppressor such as one of the Midnite Solar MNSPD that are used for solar lightning protection would probably work, but that is about $100 and only a guess.
There are quite a few solutions that I know of for using both FLA and LFP combined (LFP system charges the FLA that is used for starting). However, none are super cheap.
I'm sure there is a solution for using an LFP as a starting battery, other than buying a battery rated for starting.
This is actually a fairly recent problem with all the "drop-in" LFP solutions that have a built in BMS. Its a race to the bottom for price and I think many of those solutions are paring down the BMS and removing the surge protections. Thus the "don't use as starting battery" warning. With the old DIY solutions many of the BMS were much more robust and handled engine starting no problem.
On another note, I did buy a LFP staring battery for one of my quads. It has been AMAZING as the engine spins way faster and I haven't had to deal with a dead battery yet. However, I don't use that quad in the winter.
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gcrank1
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# Posted: 8 Jan 2025 09:57pm
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Never had one but....wouldnt one of those 'battery isolators' be useful for this to just keep the LFP topped up without back feeding the LA system?
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travellerw
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# Posted: 8 Jan 2025 10:54pm
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Quoting: gcrank1 Never had one but....wouldnt one of those 'battery isolators' be useful for this to just keep the LFP topped up without back feeding the LA system?
They make ones that go that way (or even both ways).. but this is where issues happen. LFPs are alternator killers (or belts). They suck up amps and keep the alternator operating at max for much longer than they were designed. Eventually cooking it.
IMHO you are better using a LFP charger on the 120V side of the generator and charging your LFP that way.
A common design on boats was that LFP was house bank only and you had 1,2 or 3 FLA starter batteries (1 for each ICE, in a catamaran 2 for the propulsion motors and 1 for the generator). Battery combiners were used to ensure the solar and LFP system kept the FLA batteries charged as you could sit for weeks at a time. Usually we would wire an emergency solenoid to a switch at the help. This would allow you to connect the house LFP to the starter batteries in an emergency.
Of course, like anything there is a few ways to do things depending on the scenario. Thats what we did on boats, but in an RV its not so crucial to be able to start a motor. So I would probably rethink that design on an RV.
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Nobadays
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# Posted: 9 Jan 2025 02:07pm
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Quoting: travellerw The second problem with a starter is the collapsing field when its de-energized. The magnetic field in the starter collapses generating a voltage spike that is fed back through the system. That voltage spike can absolutely kill a BMS
Wow... I did not know this! I hate to admit it but I wonder if this is what caused the first LiTime battery to swell, and the BMS to show 52% SOC one day and 100% SOC the next... with no charging being done. Maybe it was me starting the generator off that battery that fried the bms. Fortunately I did not start the generator with this new LFP battery... and won't!
Looks like I'm running a wire to the chassis battery.
Thanks for the great explanation!
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gcrank1
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# Posted: 11 Jan 2025 08:36pm
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As I recall Steve_S has commented that many (and I would guess especially the lower tier lfp bats) say they have a bms but don't and if they do it is gonna be a bargain basement unit.
So, we expect whatever quality bms we have, or don't have, to be our Last Line of Defense....hmmm, two is one and one is none comes to mind.
Wonder if there is some kind of 'bolt-on' the bat aux bms?
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paulz
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# Posted: 12 Jan 2025 07:15am - Edited by: paulz
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Ha, that probably explains my new cheapo. But it’s started my engines, including the Onan multiple times yesterday. At the moment I have it augmenting my cabin bank and adding noticable juice.
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Nobadays
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# Posted: 12 Jan 2025 09:25am
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The LiTime is actually one of the better Chinese ready made batteries and the BMS seems legit. That said as travellerw pointed out, they are usually not protected from a back feed power surge.
Whether this is what killed my first battery or not, I think it not wise to use it as a Starting Battery especially since the manufacturer says not to.
Now to crawl under the van and find a good route to string a wire to the chassis battery. That I'm not looking forward to!
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DRP
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# Posted: 12 Jan 2025 10:07am
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Not sure if it would apply or help, just dusty memory and a quick google;
"A flyback diode is any diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. It is used in circuits in which inductive loads are controlled by switches, and in switching power supplies..."
Do check line loss on any long cranking lead, they get big pretty fast if there is any distance between battery and starter.
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travellerw
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# Posted: 12 Jan 2025 12:04pm
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Quoting: DRP Not sure if it would apply or help, just dusty memory and a quick google;
A flyback diode (essentially a diode installed backwards across the inductive load) is the solution for small relays. In super simple terms it allows the spike to dissipate backwards across the inductive load and not propagate through the circuit.
Unfortunately, its not really a solution for a big system like a motor starter. Sometimes they are used on the starter solenoid, but not robust enough for the starter. The only real solution I know of is to have the BMS designed to handle that spike. Its not uncommon for the spike to be 100-120V DC when the field collapses. So the solution must be robust enough to clamp a voltage spike of that size.
This is an area of electronics where I'm not very familiar. I don't know how you would suppress a voltage spike like that if your BMS wasn't designed for it.
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paulz
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# Posted: 13 Jan 2025 10:12am
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Crow eating department:
Although the 12 page manual that came with my Lfp has a half page of do nots (don’t connect plus and minus terminals, uh thanks), there’s no mention of starting engines. So I emailed their tech dept. They replied that though it’s worked for me, really shouldn’t do that, for the reasons stated here. I wrote back that they should say that in the book like others do. Mine still seems to be ok, still hooked to the cabin bank and holding 13.3. Maybe my engines were a small enough draw.
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gcrank1
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# Posted: 13 Jan 2025 11:31am
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Im pretty sure you are Ok (and that is as far as my guarantee goes).
Im also thinking that as long as a jumpered LFP goes to a LA that goes to the starter that LA will act as a 'buffer' preventing that spike.
This buffer thing happens with a mc that runs a cdi off an alt (my Yam SR 500 and likely most 'modern mc), the LA bat prevents spikes from the alt killing the cdi. I wired in a fairly large capacitor which I guess absorbs the spikes.
Wonder if that same concept has any application here?
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Nobadays
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# Posted: 17 Jan 2025 06:17pm
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Well I was out hiking this morning and had an epiphany! Rather than run an 8awg wire from the back of the van all the way to the chassis battery up front, I'm just going to install a small generator starting battery near to my lithium ones. The Onan generator has a 224cc engine so I found a 12v/10ah/180CCA, AGM battery intended for motorcycles, ATV's, generators, etc. Looking through the reviews I found several people using this same battery to start larger generators... one in particular was 7500w which I found to have a 459cc engine. Perfect!
The charging wire from my alternator (battery isolator) already terminates at the Victron dc/dc charge controller that is mounted less than 18" from the LFP batteries. I can grab power off that to charge the 12v agm. I think there will be space either very near the Victron CC or for sure next to the LFP batteries to mount the agm.
Problem solved!
Thanks all!
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