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max_w101 12-14-2019 12:33 AM

Charging outboard battery onboard
I bought a 12V 135ah Lithium Iron LiFePo4 Deep Cycle Rechargeable Battery with built in BMS off eBay for my electric outboard. Itís meant to be a ďdrop inĒ replacement for lead acid car battery. Normally I charge at home with supplied battery charger that plugs into its own port and all is well. What is the best way to charge onboard?

1) plug the supplied charger into my inverter


2) use jumper leads to connect the discharged battery in parallel to my house bank?

I assume 2 is more efficient as the voltage is not going through a step up / step down path, but not sure if itís good for the rest of the batteries etc

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john61ct 12-14-2019 02:28 AM

Re: Charging outboard battery onboard
The second option is indeed better for efficiency, but from ICE sources that's not so critical.

Watch out for high current flows when paralleling the two when voltage is different by more than say half a volt.

Best would be a user-adjustable DCDC charger like Sterling BB series.

max_w101 12-14-2019 02:46 AM

Re: Charging outboard battery onboard
Thanks for fast reply. When you say be careful where voltage differs, do you mean there could be damage to me or equipment? Surely an alternator charging flat batteries would have a voltage difference of more than half a volt? Or is it that a battery bank would deliver more amps than an alternator, in which case I would assume the inbuilt BMS would handle this?

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john61ct 12-14-2019 06:04 PM

Re: Charging outboard battery onboard
Presumably the charge source is not active while you are connecting / disconnecting the batteries to be paralleled.

Depending on the CAR of the chemistries involved, connecting directly a very low SoC thus low voltage pack, to one at a very high SoC / voltage, could lead to a very sudden flow of hundreds of amps.


The usual advice, is to make sure the two banks are at a very similar voltage before connecting them together.

A DCDC charger in between accomplishes many things of value, and handling that issue safely is one of them.

A BMS may or may not include over-current protection, and even if it does that should only be used for failsafe/backup, rare-to-never accidents

not day to day operations.

john61ct 12-14-2019 06:42 PM

Re: Charging outboard battery onboard
If you are happy with the **voltage** level of the charge sources, the another way to limit the current is with an Echo-Charge, in this case to 15A.

The EC will drop the source voltage by ~0.4V but in this case that's likely a good thing, I use about 13.8V for 4S LFP. The maximum cap is 14.4V.

john61ct 12-14-2019 06:45 PM

Re: Charging outboard battery onboard
Find out what the "start-balance" voltage setpoint is on the internal BMS, and if it can be adjusted.

You need to "float" at the end of the charge cycle, or at least above that V point, long enough to finish getting the cells balanced.

So while you're at it, see if there's a way to see voltages at the per-cell level so you can verify that.

The higher that voltage needs to be the more stressful to the cells, so best to do that phase at a low current.

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