Why can’t I parallel an empty lifepo4 with the full lifepo4 house bank

[Fuss]

My Torqeedo has a 24v 50ah lifepo4 with built in bms. Can I simply parallel this to the lifepo4 house bank. I know that this is not supposed to be ok and that the voltages should be nearly equal before paralleling lifepo4, but what happens when a car with a flat lifepo4 battery gets jumped by a full lifepo4 battery, it’s a similar scenario right?

I’m thinking of having 2 x 50ah lifepo4s for the dinghy and when one is flat connecting it to the lifepo4 house bank and disconnecting the full one from the house bank and replacing the flat one in the dinghy.

Has anyone done this?

[smac999]

The only issue may be huge currents between them when connected. You could put a breaker Inbetween to be safe.


Only in series would they need to be the same before connecting.


A dc to dc converter may be a better option.

[Fuss]

Thanks for the reply, if there is a huge inrush, I guess I could make the cable long to give some voltage drop which would limit the inrush and not stress the built in bms too much

[ThereAndBack]

Originally Posted by Fuss

Thanks for the reply, if there is a huge inrush, I guess I could make the cable long to give some voltage drop which would limit the inrush and not stress the built in bms too much

It isn't going to be okay.

My suggestion is to get a 30amp DC to DC charger and use that to charge your flat battery off the house bank.

[ThereAndBack]

Originally Posted by smac999

A dc to dc converter may be a better option.

Agree

[Fuss]

Thanks for the reply, can you tell me why? Does that also mean that it is not possible to jump start a car with a lifepo4 battery installed.

[goboatingnow]

Originally Posted by Fuss

Thanks for the reply, can you tell me why? Does that also mean that it is not possible to jump start a car with a lifepo4 battery installed.

Because worst case 1000s of amps could flow from the fully charged lithium to the discharged lithium’s. Lithium’s are like Labrador’s. They never know when to stop.

Your cables will glow like little suns !!!

[goboatingnow]

Originally Posted by Fuss

Thanks for the reply, can you tell me why? Does that also mean that it is not possible to jump start a car with a lifepo4 battery installed.

With controls on the battery no.

[Fuss]

Ok, thanks, so this means that anyone with a lifepo4 battery in their car could burn it to the ground …are their warnings about this, I tried looking on the web but I don’t find any at all.
Could anyone point me to these warnings?
Thanks John

[goboatingnow]

Originally Posted by Fuss

Ok, thanks, so this means that anyone with a lifepo4 battery in their car could burn it to the ground …are their warnings about this, I tried looking on the web but I don’t find any at all.
Could anyone point me to these warnings?
Thanks John

Some lithium BMSd have over current protection .

But if you tried to jump start a small lithium from a big one. Nope definitely not a good idea.

[PaulCrawhorn]

Going from LFP to lead is OK.

The target battery determines the current drawn.

LFP will always try to pull more than is healthy, and there needs to be a current limiting device - not just over-current protected, in between.

Yes, resistors or a certain length certain size AWG cable will in theory work but risky, requires more brain than most got, and only good for that pairing.

Finally, charging from stored energy is foolishly inefficient. Have an active charge source do it, genset, wind, solar, alternator.

[dmalcol1]

Following this thread

[Mirage35]

I'm not sure what problem you're trying to solve. I charge my Torqeedo battery from the house bank using a charger that limits the current. Takes longer but nothing overheats.

[Adelie]

Originally Posted by smac999

The only issue may be huge currents between them when connected. You could put a breaker Inbetween to be safe.


Only in series would they need to be the same before connecting.


A dc to dc converter may be a better option.

Do not parallel full and discharged batteries. The full one will recharge the empty one. In doing so you will get very high currents which will not be good for either battery.
More importantly you may exceed the capacity of the cabling and/or connectors, overheating and starting a fire being the specific concern. Lengthening the cables will induce some voltage drop but not enough to make a significant difference.

[SamH]

In the real world...
It will works just fine.. Been there, done that.
Even though the ir is low, so is the initial differential voltage, and the diff wil decrease in ms, if not microsecs.
Did it with 2p4s 400 Winston cells. (They are 10C rated, so 2x4000A max)
No issues what so ever. The BMS with 500A relays didnt even blink.

Wouldnt do it on a regular basis though. Sparks will fly..