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

[PaulCrawhorn]

Originally Posted by Fuss

melted cables and blackened fingers?

aka Kentucky Fried Fingers

be careful out there...

[YANDINA]

The Trollbridge 12x24-Lithium can be used with a pair of 12 volt LiFePO4 batteries up to 100 amp-hour to charge them from a 12 volt alternator or shore power. It automatically switches them in series when you turn on the trolling motor. Otherwise they sit normally in parallel. They are rated for alternators up to 100 amps and trolling motors up to 85 amps. The starting battery can be lithium or lead-acid.

When in series the batteries are being discharged equally and since they were in parallel prior to switching their charge levels will be depleting equally so when restoring to 12 volts the inrush current is negligible.

When first installed or if a battery is changed, there may be sparks and current when first connected however current is limited to a safe level by using the required limitations on wire gauge and length which add 0.008 ohms of series resistance.

After a fishing trip, most customers find the batteries are fully charged when they get back to the trailer. If needed a portable charger can be clipped on the starting battery to top them off saving the co$t of a dual 12 volt marine grade charger.

The Trollbridge 36-Lithium works in a similar manner for 36 volt trolling motors.

[PaulCrawhorn]

Originally Posted by YANDINA

The Trollbridge 12x24-Lithium can be used with a pair of 12 volt LiFePO4 batteries up to 100 amp-hour to charge them from a 12 volt alternator or shore power

How about if all banks and sources are already at 24V?

Could your simpler combiners be used with calculated-resistance wiring rather than going for DC-DC current limiting?

[YANDINA]

Originally Posted by PaulCrawhorn

How about if all banks and sources are already at 24V?

Could your simpler combiners be used with calculated-resistance wiring rather than going for DC-DC current limiting?

The Trollbridges work from a 12 volt alternator to charge 24 or 36 volt banks so the source bank can only be at 12 volts. The 24 or 36 volt banks to be charged are automatically connected in parallel for charging and re-connected in series by the Trollbridge to run the 24 or 36 volt load. So the bank voltage is under the control of the Trollbridge. This is why the Trollbridges are not compatible with a 24 or 36 volt battery, only with multiple 12 volt batteries.

Yes, we have used "calculated-resistance wiring" for 30 years to limit inrush current to safe levels. You are correct, the Trollbridges have the "simpler combiners" already built in.

[PaulCrawhorn]

Yes, I am not talking about your Trollbridges anymore.

My question is, for an all 24V system, could your combiner models combined with right-sized wiring, substitute for using DCDC in this use case?

i.e. connecting a depleted LFP pack to the larger "mothership" LFP House bank at 100% Full

to limit the current allowed, preventing too high an inrush spike...

[sailingchiro]

I hooked my cells together in order to top balance them with 2 gauge wire and it went up in smoke and set off my smoke detector. My wife was not happy.
As many have said, you will need to limit the current (amps). This you will either need to increase the resistance or use a dc to dc charger.

Try using a power resistor such as a 200 watt but you will need to determine the resistance to reduce your current.

[Fuss]

So far, I have been using 16mm wire and there has been no signs of melted cables or damaged batteries. I’m only at the early testing stages though and so far it looks like the 16mm wire will do it. For safety, I probably will give it some more length.

[Fuss]

with one battery full and the other at 60% I am still not getting sparks and warm cables

[Adelie]

Full and 60% was not the original question. LFP has a pretty flat voltage curve thru the middle 70% so there probably isn’t much voltage differential which is what you need for sparks. What you should be measuring is amps in the cabling.


The original question was full and empty. Full to 10% might give you a spark. Better to measure amps.

[YANDINA]

Originally Posted by sailingchiro

I hooked my cells together in order to top balance them with 2 gauge wire and it went up in smoke and set off my smoke detector. My wife was not happy.
As many have said, you will need to limit the current (amps). This you will either need to increase the resistance or use a dc to dc charger.

Try using a power resistor such as a 200 watt but you will need to determine the resistance to reduce your current.

Hmm, If your 2 gauge wire went up in smoke it sure looks like you had them shorted in series instead of parallel.

[YANDINA]

Originally Posted by PaulCrawhorn

Yes, I am not talking about your Trollbridges anymore.

My question is, for an all 24V system, could your combiner models combined with right-sized wiring, substitute for using DCDC in this use case?

i.e. connecting a depleted LFP pack to the larger "mothership" LFP House bank at 100% Full

to limit the current allowed, preventing too high an inrush spike...

The built-in cables on our 12 volt Combiners add 0.004 ohms which is sufficient to protect the internal switch. For smaller overloads there is a thermal shutdown at 85C that shuts it down until the temperature drops back to 75C.

We discontinued our 24 volt Combiners due to lack of sales. West Marine used to carry our 24 volt Combiner but we had to buy back about 30 due to lack of sales.

[PaulCrawhorn]

That's too bad.

Thanks for the info Ann-Marie

[goboatingnow]

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

Remember I2R heating

[CaptainRivet]

Originally Posted by 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?

Why not charge them from the active charging source?
I would just put a connector in where you can plug your 50AH outboard bat and
1) put a 1/2 switch at the output of your charge busbar or if you don‘t have your eg MPPT and then switch to outboard bat till full
2) or a Argo FET connect the connector to bank 2 output and your house to 1 and that automatically then divides the charge current between the house (bank 1) and the torquedo (bank 2)

A dc to dc charger with 30A is 250Euro….you can charge anytime
the above maybe 100…you can charge when charge sources are active

Paralleling a full and empty LFP just damaging both batteries (besides the dangers many pointed out already) and shorten their life drastically…you don‘t save anything here (you actually throwing money out of the window) and there is no shortcut

[CapnCookinOz]

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.

No, sorry you are wrong.
Batteries in series - you ADD the voltages. ie 2 x 12v batteries in series gives 24v at the same available current (amps).
Batteries in parallel - the voltage stays the same, but the current available doubles (in theory). ie 2 x 12v batteries in parallel still give you a 12v supply, but more current is available