I have an unconventional question - is there any reason to disconnect a 12V LiFePo4 "house" battery from the 12V lead acid "starter" battery when the engine isn't running and the house battery is above the nominal voltage of the starter battery? (lke 12.7v) The starter battery should just be sitting there not really drawing or supplying anything right? (unless it's discharged or dyeing)
When is 12V lead/LiFeP04 Battery isolation necessary?
- Thread starter geoffire
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Current flows from high to low voltage.
FLA/AGM/GEL wants to settle to around 12.7V. LFP rarely settles below 13.2V. The LFP batteries in parallel with FLA/AGM/GEL will "float" the FLA/AGM/GEL batteries consuming a small amount of power all the time.
FLA/AGM/GEL wants to settle to around 12.7V. LFP rarely settles below 13.2V. The LFP batteries in parallel with FLA/AGM/GEL will "float" the FLA/AGM/GEL batteries consuming a small amount of power all the time.
That's a trivially low current though right? Battery tender range.
Depends on the size and condition of the starter battery. Could be trivial on a healthy battery.
True, we had the start/stop battery in our Pacifica die (the two batteries are connected most the time). First the car just disabled start/stop and I didn't care, then about a year later the starter battery started dying when the car sat a couple days. We went to replace the starter battery but it tested good so Idug in found it was the start/stop one draining it so I disconnected that. All was then ok but it seems that ordeal weakened the starter battery as we had to replace it 3 months later.
I just had the thought that assuming the alternator can handle the current and the "house" loads are shutoff below 12.7v, there is no need for a DC-DC charger or an isolator (let the BMS handle overcharge protection leaving the house loads on the alternator).
edit: I'd definitely have a manual disconnect switch (between the two and before all the loads) and flip that when the vehicle is sitting for a while.
I just had the thought that assuming the alternator can handle the current and the "house" loads are shutoff below 12.7v, there is no need for a DC-DC charger or an isolator (let the BMS handle overcharge protection leaving the house loads on the alternator).
edit: I'd definitely have a manual disconnect switch (between the two and before all the loads) and flip that when the vehicle is sitting for a while.
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That's the discussion on another thread. I'm not convinced it will be an issue, but I will test my truck to see here soon. Not discharging into the lower "shoulder" voltages avoids the worst case current draw.
Normally a motorhome would have a battery isolator to separate the house and chassis start battery. This allows you to actually start the motorhome and drive away even if you have overly discharged the house battery. If both are connected, both would run down together.
Second issue is the LFP would be continuously float charging the FLA start battery down to when they equalize at about 12.7 volts. This may use significant power that you need for other house loads.
If this does not concern you then both connected does no real harm that I am aware of.
Second issue is the LFP would be continuously float charging the FLA start battery down to when they equalize at about 12.7 volts. This may use significant power that you need for other house loads.
If this does not concern you then both connected does no real harm that I am aware of.
It actually doesn't. Even with completely avoiding the bottom leg, the vast majority of the charging of LFP occurs below the typical 14.4V for FLA - closer to 13.6V. This will cause the alternator to run at max output for the duration of the charge.
If you have thinner wires that can handle the current but result in a voltage drop, this can work to your advantage. The alternator will "see" a higher voltage than the battery, hit its peak voltage and begin tapering current.
rootusrootus
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I thought I recall seeing somewhere that the 'burning out alternator' problem is dependent on vehicle. E.g. Ford limits current on the charging circuit to a level that won't overtax the alternator.
That's on the 7-pin connection. Even then, it isn't clear if a discharge LiFePO4 battery can suck the ever living daylights out of the circuit enough to blow the 30 amp fuse and/or trash the 30 amp relay.
rootusrootus
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Huh. Interesting. Well, I guess I'm going to find out
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fratermus
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I think OP is saying that a smaller voltage delta between the house battery and chassis would reduce charging current demand on the alternator. Doesn't the delta / resistance == charging current? Working at the edges of my understanding here, so please correct me if I am confused as usual.
I don't know if any such reduction would be enough to make a practical difference, given the very low internal resistance of Li banks.
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