So I had been running two batteries in parallel on my b6100, a group U1 ‘riding mower battery’ in the tray, and a group 16 ‘powersports’ battery on the left hand loader subframe. Here’s a pic of how i mounted that 16 before we move on:
Anyway, the U1 died and needed to be replaced. I could have done the ~$34 thing and just bought another U1 riding mower battery from Walmart, but i wanted to turn it into an experiment instead. Ive been transitioning from lead acid to lifepo4 in both my RV and my house (my small house is 100% off grid with solar and batteries). While generally they are sold as ‘deep cycle’ and not appropriate for engine starting, they DO sell starter battery versions woth the caveat that theyre VERY pricey, like $300-800 pricey. Perhaps justifiable on a sports car for their very light weight, but not justifiable on a B6100!!
However.. car-sized batteries arent the ONLY starting batteries. There are 'powersports' / motorcycle lifepo4 starting batteries as well, and on Amazon those are getting down into the $40-60 range! They are 'rated' for 200-300 'cold cranking amps' in that price range, and are only 2-6ah of capacity! I assume they are simply some tiny but otherwise 'normal' lifepo4 cells hooked to a relatively monstrous BMS, and cycle life be damned (but still claim 2000-8000 because who will stop you!?). I have not opened one to verify. But I found a ~$40 2ah model rated for 200a and bought it.
I paid ~$40 with a coupon
The model i bought, i found disappointing for this application. My little tractor takes about 130-180a in current conditions (it was 70f) and the battery would only crank it intermittently. I do not have an inrush meter but i know that if one were to scope a starter motor's current waveform it has an enormous but very brief spike before quickly dropping to 'steady' cranking amps. Unfortunately in this case, the bms electronics are fast enough to catch that and disconnect before the current drops off to the level the BMS is rated for. Even when the bms makes it across that jump and cranks the engine, I only get 1-2 good seconds of cranking before the BMS trips again. Put one way, it is WAY LESS of a cranking battery than the $37 lithium 'jump pack' i bought a couple of months ago. So, disappointing. BUT!
But i never really intended to put a 2ah battery alone in my tractor (a U1 is perhaps ~20-35ah depending on which version of a U1). I wanted a bit more margin for error than that, such as if i run the thing out of diesel and have to crank the engine to purge the air from the injection lines before it will start again. So I had the thought of paralleling a larger ah battery to the tiny motorcycle battery so that it would 'contribute' to some extent during cranking, but mostly 'add capacity' to the starter battery by transferring charge to it pretty rapidly, giving me many good long cranking attempts, only perhaps requiring some decent pauses in between.
I paid ~$41
For this purpose i bought an 18ah battery for ~$41. I figured if this didn't work out it would go in one of the little electric cars for my 2 & 5 year old daughters, lol. It has a 20a continuous rated BMS, and supposedly allows 40a for up to 5 seconds. With this whole engine-cranking thing we are spending all our time not in the continuous rating, but in that 1s-5s duration rating that not many rock-bottom-money Amazon listings even mention. There is not even ROOM for a 200a bms in the 200a rated battery, so i believe the 'cranking amps' number these starter battery listings are using is really something at or under that 1-5 second rating we see specified on higher quality products. Anyway, I put a 100amp load tester to this 18ah battery and it consistently immediately tripped the bms. However, my theory for why the bms wouldnt trip during engine cranking was because my parallel connection to the other battery would be no larger than 12ga, which would 'throttle' the current sufficiently to stay in that 1-5 second rating region until the cranking was over. So, i hooked the 100a load tester through some 12ga wire and the 18ah battery did 75 amps for a solid 5+ seconds, and i lost my nerve and let off to see if my 12ga was getting too hot, before the bms actually tripped. It did far more than the 40a it claims to allow. So far so good!
Once the two batteries were in parallel in the tractor with 12ga wiring in between, it DID work out basically as i thought, with the caveat that the 2ah bms still occasionally tripped. When it didn't, the batteries cranked the engine fast and plenty long enough, and i didn't have the patience to see how many 'good long cranks' i could get out of it but it was definitely enough. I measured both batteries together to crank the engine fast at 180a with 75a of that coming from the 18ah, and the 2ah alone (18ah disconnected) cranks the engine a bit sluggishly at 133a.
I considered my idea to be proven viable, with the exception that the 2ah 'starter' battery (really just its internal bms) is slightly undersized for this task. So, i returned it and replaced it with a slightly larger one for a few bucks more (i started with a group 4, replaced it with a group 7). Heres the one i bought, but ive since found cheaper at $47.
I paid $56
But id buy this bigger one for $48 if i had to do it again today.
Anyway, the U1 died and needed to be replaced. I could have done the ~$34 thing and just bought another U1 riding mower battery from Walmart, but i wanted to turn it into an experiment instead. Ive been transitioning from lead acid to lifepo4 in both my RV and my house (my small house is 100% off grid with solar and batteries). While generally they are sold as ‘deep cycle’ and not appropriate for engine starting, they DO sell starter battery versions woth the caveat that theyre VERY pricey, like $300-800 pricey. Perhaps justifiable on a sports car for their very light weight, but not justifiable on a B6100!!
However.. car-sized batteries arent the ONLY starting batteries. There are 'powersports' / motorcycle lifepo4 starting batteries as well, and on Amazon those are getting down into the $40-60 range! They are 'rated' for 200-300 'cold cranking amps' in that price range, and are only 2-6ah of capacity! I assume they are simply some tiny but otherwise 'normal' lifepo4 cells hooked to a relatively monstrous BMS, and cycle life be damned (but still claim 2000-8000 because who will stop you!?). I have not opened one to verify. But I found a ~$40 2ah model rated for 200a and bought it.
I paid ~$40 with a coupon
The model i bought, i found disappointing for this application. My little tractor takes about 130-180a in current conditions (it was 70f) and the battery would only crank it intermittently. I do not have an inrush meter but i know that if one were to scope a starter motor's current waveform it has an enormous but very brief spike before quickly dropping to 'steady' cranking amps. Unfortunately in this case, the bms electronics are fast enough to catch that and disconnect before the current drops off to the level the BMS is rated for. Even when the bms makes it across that jump and cranks the engine, I only get 1-2 good seconds of cranking before the BMS trips again. Put one way, it is WAY LESS of a cranking battery than the $37 lithium 'jump pack' i bought a couple of months ago. So, disappointing. BUT!
But i never really intended to put a 2ah battery alone in my tractor (a U1 is perhaps ~20-35ah depending on which version of a U1). I wanted a bit more margin for error than that, such as if i run the thing out of diesel and have to crank the engine to purge the air from the injection lines before it will start again. So I had the thought of paralleling a larger ah battery to the tiny motorcycle battery so that it would 'contribute' to some extent during cranking, but mostly 'add capacity' to the starter battery by transferring charge to it pretty rapidly, giving me many good long cranking attempts, only perhaps requiring some decent pauses in between.
I paid ~$41
For this purpose i bought an 18ah battery for ~$41. I figured if this didn't work out it would go in one of the little electric cars for my 2 & 5 year old daughters, lol. It has a 20a continuous rated BMS, and supposedly allows 40a for up to 5 seconds. With this whole engine-cranking thing we are spending all our time not in the continuous rating, but in that 1s-5s duration rating that not many rock-bottom-money Amazon listings even mention. There is not even ROOM for a 200a bms in the 200a rated battery, so i believe the 'cranking amps' number these starter battery listings are using is really something at or under that 1-5 second rating we see specified on higher quality products. Anyway, I put a 100amp load tester to this 18ah battery and it consistently immediately tripped the bms. However, my theory for why the bms wouldnt trip during engine cranking was because my parallel connection to the other battery would be no larger than 12ga, which would 'throttle' the current sufficiently to stay in that 1-5 second rating region until the cranking was over. So, i hooked the 100a load tester through some 12ga wire and the 18ah battery did 75 amps for a solid 5+ seconds, and i lost my nerve and let off to see if my 12ga was getting too hot, before the bms actually tripped. It did far more than the 40a it claims to allow. So far so good!
Once the two batteries were in parallel in the tractor with 12ga wiring in between, it DID work out basically as i thought, with the caveat that the 2ah bms still occasionally tripped. When it didn't, the batteries cranked the engine fast and plenty long enough, and i didn't have the patience to see how many 'good long cranks' i could get out of it but it was definitely enough. I measured both batteries together to crank the engine fast at 180a with 75a of that coming from the 18ah, and the 2ah alone (18ah disconnected) cranks the engine a bit sluggishly at 133a.
I considered my idea to be proven viable, with the exception that the 2ah 'starter' battery (really just its internal bms) is slightly undersized for this task. So, i returned it and replaced it with a slightly larger one for a few bucks more (i started with a group 4, replaced it with a group 7). Heres the one i bought, but ive since found cheaper at $47.
I paid $56
But id buy this bigger one for $48 if i had to do it again today.
