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Replaced 75AH SLA with 320AH LiFePo4 in my RV, now this…

cmpct_cmpr

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Sep 28, 2020
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Hi folks,

with the help of Will‘s videos and this forum, I was able to build a 12V 320AH LiFePo4 battery bank.

I replaced the old sealed Lead Acid 12V 75AH battery in my RV, a 2001 Knaus Sun Traveller, based on a Fiat Ducato.

I have 200W of solar Panels on the roof, that charges the battery with a Votronic MPPT controller.

The RV comes with built-in electrobloc that handles all matters charging and using the battery.

The device used to parallel connect the car‘s starter battery and the RV battery on certain occasions, I think when both batteries had a similar SOC when charging by solar, by external 220V AC, or by alternator while driving.

Everything is so much better with my DIY 320 AH battery bank - tbh it is oversized at this moment (but planning to get a compressor-fridge later this year)…

BUT I had the electrobloc behaving in a strange way a few days ago: At about 80% SOC it put both batteries into parallel, immediately disconnected them again only to parallel them again, disconnect, connect etc very quickly - I could hear the relay clicking continuously. It stopped when I started the engine.

It doesn’t happen at a higher SOC, I can‘t tell lower SOCs yet.

Can you recommend some modification to the system?
 
Your DIY LFP BMS may respond to the automatic paralleling of the batteries as a short and engaged short circuit protection cutting the LFP out of the system. It likely has a 5 second reset following a short.

1: Parallel
BMS short circuit protection
Un-parallel
BMS drops short circuit protection
System sees coach battery again, goto 1:

It likely doesn't happen at higher SoC and engine running because the chassis battery voltage is much closer to the LFP, and the surge when paralleled is lower.

Recommend you disable the paralleling of the system and only do it when you think it's necessary. I'm guessing it's a solenoid that completes the circuit joining the two battery (+) conductors. You could install a manual switch on the solenoid coil wire. When the switch is off, no parallel. When the switch is on, it will parallel in accordance with its normal operation.
 
Thanks for your reply.

It is not the BMS though, there‘s no short in the logs, and the sound of the relay comes from the electrobloc.

I can‘t disable it without going into the electrobloc, which I‘d rather not do.

In my fantasy, the paralleling of the two batteries causes a high amperage, so the bloc is un-paralleling the batteries. This is a pure guess though. My hope is that I can add some piece of technology outside the electrobloc, between the batteries, to stop that on/off thing.

A switch to open or close the paralleling cable seems kind of a last resort to me.
 
I would suggest installing a dc-dc between alternator and the calira.
That also would limit the current through the wires.

The ducato x230 is a little older so probably doesn’t have a huge alternator so better to save it by limiting the current.

Personally, i prefer to bypass the calira’s (and schaudt ebls) all together since there internal chargers aren’t really sufficient for a huge battery bank and has a too high floating voltage (13.8v).
In case of a gas refrigerator you have to look into the schematics to see what the best options are regarding aes, but for a compressor refrigerator its easy, since you don’t have to deal with aes signals anymore.

For the overseas people: here in Europe many small rv’s are based on the Fiat Ducato diesel engjnes (the later x250 series is known as promaster in the usa) and schaudt, calira, arscilicii and cbe are the most used by rv builders as all in one devices (charger, fuse box and relays)
 
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