Parallel Battery Monitoring

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grumbulbum asked Nov 8, '20

Parallel Battery Monitoring

Hi,

I have 2 x 12V 240AH AGM batteries wired in parallel that are used to start my starboard engine and also double up as the house batteries on my boat.

One of the batteries recently started to become very hot and was constantly drawing 16amps from the charger, upon removing and testing, it still reported plenty of CCA, Voltage and current draw were still good but the casing was showing signs of deforming from heat stress.

I've since decided to replaced both batteries to be safe and now looking to add battery monitoring and solar charging as none currently exists in the boat.

In the picture attached/below is how I believe I should wire up 2 x BVM712 and a MPPT100/20 so I can monitor each battery independently.

I would expect I need to set each 712 up as 240AH and will each one report correct SOC, Current Draw/Charge for each battery rather than as a battery bank?

While I could monitor the bank with only 1 x BVM712, I want the ability to monitor each battery independently

It may be an overkill but am I going about this the right way?

Thanks Paul

MPPT Controllers BMV Battery Monitor

712-setup.jpg (78.5 KiB)

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Answer 1 · 2020-11-09T22:14:24Z

Vangelis Beligiannis answered · Nov 09, 2020 at 10:14 PM

You can use two bmv if you like. Pay attention on the connection of the cable before departing for the load. You have to connect each cable from the positive pole to abusbare and then depart for the load. The same for the negative. Otherwise more current Will pass from one battery and this will unbalance everything. (maybe you know this but the diagramm does not clarify it). If you have a GX device you also need to declare the shunts as 1000 Amps each to get the right results (because the loads are destributed to two batteries)

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Answer 2 · 2020-11-10T01:21:47Z

Justin Cook answered · Nov 10, 2020 at 01:21 AM

Hi @grumbulbum, welcome to the Community!

This is definitely not the correct way to connect your system; you should (properly) parallel-connect these two batteries to form one bank, which you will then be able to accurately monitor.

The same goes for your proposed charging method: you should not attempt to connect a single solar controller to two separate batteries, but rather parallel-connect those batteries to form a single bank and then charge that bank as one.

So long as the two batteries are properly parallel-connected, this is the best and only reliable and accurate way to do what you're trying to do: both the BMV and the MPPT require a single reference voltage to function properly, so trying to treat the batteries as separate even though they're not will net you nothing but trouble, inaccurate charging, and inaccurate capacity readings.

See Victron's book Wiring Unlimited for clarification on the 4 (and only 4) acceptable ways to parallel-connect batteries; from your brief description it sounds like the batteries you just replaced were likely improperly connected, thus the premature failure of one.

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