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

[rramsden]

It's physics.
You can put batteries in series, no problem.
Parallel is a different issue.
Batteries in series offer the same available amperage, just more depth. Batteries in parallel double the available amperage, and usually twice the voltage. 2 24s in parallel offer 48 volts.

Major change in the equation.
Lots of non trivial math. You don't don't want to go there.

[Fuss]

Originally Posted by 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.

Wouldn't do it on a regular basis though. Sparks will fly..

Thanks for this information. I am going to try some experiments with two 24v lifepo4s and see what happens.
If I find that the BMS can handle it and that there is no damage then I will use my outboard batteries as a sort of extension of the house bank...so the house bank will get 50ah - 100ah more capacity when the outboard is not in use... and when the outboard is in use, 50ah - 100ah less capacity.

[hzcruiser]

What does Torqeedo recommend to charge their 24V batts from a 24V source?

[Fuss]

Originally Posted by hzcruiser

What does Torqeedo recommend to charge their 24V batts from a 24V source?

You can use any battery you like, they supply a 24v 100ah lifepo4 with charger but there is no restriction, some people use a rolls lead acid 50ah, as a cost saving measure as the 100ah lifepo4 is quite expensive from Torqeedo. (of course this only works if you are travelling just to dock and back)
(Torqueedo cruise 2kw is the motor I have...this can be powered by anything the user wishes as the battery is not part of the motor)

[PaulCrawhorn]

Originally Posted by Fuss

I will use my outboard batteries as a sort of extension of the house bank...so the house bank will get 50ah - 100ah more capacity when the outboard is not in use... and when the outboard is in use, 50ah - 100ah less capacity.

This is an excellent goal, but to care for the cells, and also to be safe, make sure you put a 24V to 24V DC-DC charger in between the two sub banks for current limiting when recharging the dinghy one. I reco Sterling BB series, since the charge profile can be user custom adjusted.

For efficiency in a mostly-solar context, do not charge from stored energy, have an active source running when recharging the dinghy sub-pack, even if the "mother sub-pack" is at high SoC and much bigger than the dinghy one.

If possible, size the amp capacity of the DC-DC charger so that you could go to 0.8C to 1C, when fast charging was desired, but by default keep it set at half that rate for cell longevity. Another great feature of the Sterling line.

[Robhwa]

Originally Posted by rramsden

It's physics.
You can put batteries in series, no problem.
Parallel is a different issue.
Batteries in series offer the same available amperage, just more depth. Batteries in parallel double the available amperage, and usually twice the voltage. 2 24s in parallel offer 48 volts.

Major change in the equation.
Lots of non trivial math. You don't don't want to go there.

...careful! This is not correct. Wiring two 12V, 100A batteries in series, will produce a 24V 100A combination. Wiring the same two 12V 100A batteries in parallel will produce a 12V 200A combination. You can't get more volts and more amps with any combination of two batteries, though that would be nice. Also be careful, as some common LiFePO4 batteries (notably Renogy) can't be wired in series or the BMS will cut everything off. Generally, all batteries can be wired in parallel. Here is some good info:

https://dakotalithium.com/2018/12/07...xoCWFAQAvD_BwE

Also Will Prowse has some good information...this is the guy that tears the batteries and systems apart and thoroughly tests them, often to the detriment of companies that cut corners...

https://www.mobile-solarpower.com

[jacktheflyer]

You might like to have a look at the following website. https://www.zwerfcat.nl/en/open-hybrid-bms.html
Scrubby

[hzcruiser]

Originally Posted by Fuss

You can use any battery you like, they supply a 24v 100ah lifepo4 with charger but there is no restriction, some people use a rolls lead acid 50ah, as a cost saving measure as the 100ah lifepo4 is quite expensive from Torqeedo. (of course this only works if you are travelling just to dock and back)
(Torqueedo cruise 2kw is the motor I have...this can be powered by anything the user wishes as the battery is not part of the motor)

OK, so how do _you_ then recharge those external batts if they're depleted?

[Djragan]

My cat is all electric, house/propulsion bank is 18Kwh of LiFeP04(24V), dingy uses a 24 volt Protruar with 72Ah 24V LiFeP04 battery. In the morning(before solar panels kick in) house bank is 26.6V fully discharged dingy battery is usually 24 V. I parallel them through a 2M long 6mm2 cable. I make sure to disconnect dingy battery before sunset so it retains full charge. Has worked perfectly for 2 years, no problems. No spark at connection make/break so no huge surge apparent. There is a 30A fuse in the jumper cable, it has never blown.

[hzcruiser]

Originally Posted by Djragan

My cat is all electric, house/propulsion bank is 18Kwh of LiFeP04(24V), dingy uses a 24 volt Protruar with 72Ah 24V LiFeP04 battery. In the morning(before solar panels kick in) house bank is 26.6V fully discharged dingy battery is usually 24 V. I parallel them through a 2M long 6mm2 cable. I make sure to disconnect dingy battery before sunset so it retains full charge. Has worked perfectly for 2 years, no problems. No spark at connection make/break so no huge surge apparent. There is a 30A fuse in the jumper cable, it has never blown.

Exactly what I would expect and hence why I think this should work just fine in the OP's case: the voltage difference between house and Torqeedo batt is small enough to only cause a current that is "ok" for it. The first time doing that I would, of course, check that the cables and connectors are not getting too warm immediately after paralleling them.

I have a DC clamp meter that would tell me the exact number, but not everyone has that...

[Fuss]

Originally Posted by Djragan

My cat is all electric, house/propulsion bank is 18Kwh of LiFeP04(24V), dingy uses a 24 volt Protruar with 72Ah 24V LiFeP04 battery. In the morning(before solar panels kick in) house bank is 26.6V fully discharged dingy battery is usually 24 V. I parallel them through a 2M long 6mm2 cable. I make sure to disconnect dingy battery before sunset so it retains full charge. Has worked perfectly for 2 years, no problems. No spark at connection make/break so no huge surge apparent. There is a 30A fuse in the jumper cable, it has never blown.

Thanks, This gives me some confidence.
I have now started experimenting...I have both batteries at 26.1 volts resting.
I connected both in parallel and there was absolutely no small sparks when connected and I imagine that no current flowed between them.


later I will discharge the one with the weaker BMS to say 90% and parallel them again.


Then I'll work my way down until I see an unhealthy amp flow.

[CharlieJ]

@SamH Post 15: I agree.
Bank #1: 24kWhr bank at about 3.6VPC
Bank #2: 24kWhr bank at about 3.5VPC

Closed the remote battery switch that paralleled the two banks; current peaked at about 600+ A for less than 15 seconds and then settled to about 200A. Absolutely anti-climatic.

[PaulCrawhorn]

The current gets exponentially higher as the voltage delta is greater.

Even if no wiring gets melted, you are not doing any favours to the cells, 1-2 C would be my max current reco for longevity.

And of course once you get up to higher voltages, say over 30V much less 60V you aren't just playing around, not just blackened fingers anymore.

Play Stupid Games Win Stupid Prizes.

[Fuss]

Originally Posted by PaulCrawhorn

The current gets exponentially higher as the voltage delta is greater.

Even if no wiring gets melted, you are not doing any favours to the cells, 1-2 C would be my max current reco for longevity.

And of course once you get up to higher voltages, say over 30V much less 60V you aren't just playing around, not just blackened fingers anymore.

Play Stupid Games Win Stupid Prizes.

Thanks for the reply. Did you experience melted cables and blackened fingers? What were you doing when this happened?

[hzcruiser]

Originally Posted by PaulCrawhorn

The current gets exponentially higher as the voltage delta is greater.

[...]

Play Stupid Games Win Stupid Prizes.

So you just postulate that, even though it's in contrast with the basic Ohm's law? Better re-read your last line...

In case someone wonders, the current goes up linearly with the voltage.

But I agree, simply "playing" with large battery banks without the proper understanding or safety measures like breakers is discouraged.