Marking notes on what you've planned.

[SquirrellCook]

Finally this weekend I've installed a Victron BMV 712. I remember thinking about this many times and coming up with solutions. Unfortunately I forgot the main reason I hadn't done it. I knew the battery cables would be horrible to work on due to the fact the seat, gear lever and handbrake would all be in the way. This is on a 811D LHE Coach based conversion. So fun and games I got the shunt fitted between ground and the battery negative terminal. Did a trial start of the engine and the shunt didn't melt, so that's good. Started to feel good about the progress, and that's never good as sods law will strike you down. Bugger! How are the battery chargers going to work? The way "Murky" is set up, it has three identical battery chargers for each block of 12 volts. Two for each engine start battery and one for the habitation. The engine start being connected one charger 0 - 12 volts and the other charger 12 - 24 volts. The belief being that they would be better charged this way. Unfortunately I realised too late the shunt would not see both chargers and this is why I never fitted the BMV. The solutions I came up with was either to connect the chargers in series, or switch each to 24 volt operation and run them in parallel. I chose the latter. So apart from the obvious have I learnt anything from this expensive installation? Two new batteries would have been cheaper as they both appear to have lazy cells. 5 watts are going somewhere when it's switch off. I know some is going to the battery chargers, but it will be interesting watching the volts drop as it's parked up for the next five days.

 

[wildebus]

So your BMV-712 is monitoring the Vehicle Battery use, not the Leisure Battery use?

 

[SquirrellCook]

So your BMV-712 is monitoring the Vehicle Battery use, not the Leisure Battery use?

Two of them now Dave. So both being monitored.

 

[wildebus]

Two of them now Dave. So both being monitored.

I see
as a temporary measure to aid trouble-shooting your batteries, could you move the "Leisure" BMV to the Vehicle side? have one shunt and monitor on each of the 12V starter batteries and see what is occuring?

 

[SquirrellCook]

So early results. This morning I saw 24.04 volts on the engine start batteries after being fully charged when parked up yesterday evening. Though it did claim it was 99% charged. Deviation I think was 0.7%.
So two Varta LFD90's 50% rule gives 45 Ah at lets say 25 volts being generous. 1.124 kw sounds a lot.
It has when being watched a 5 watt constant drain. So 5 into 1125 gives 225 hours. Then divided by 24 gives 9.375 days.
So just sat doing nothing it needs charging once at week!
Conclusion, best treat it to another pair of batteries

 

[SquirrellCook]

could you move the "Leisure" BMV to the Vehicle side? have one shunt and monitor on each of the 12V starter batteries and see what is occuring?

This had crossed my mind Dave, but due to the evolution of Murky the habitation side of the electrics are not service friendly.
Though with the mid point connection it is possible to read it as volts or percentage deviation.
Once I get a chance to leave it for a while I'll get a better idea about what happens. Either way I think the batteries both have a weak cell.

 

[wildebus]

it may be worthwhile investing in a Battery Balancer/Equalizer when/if you replace the pair of starters? keep them optimally balanced should hopefully eliminate the stress on a single weaker one that could spiral downwards over time and take the other down with it? (maybe a bit overdramatic wording there )

 

[SquirrellCook]

it may be worthwhile investing in a Battery Balancer/Equalizer when/if you replace the pair of starters?

This is where the original idea of independently charging each battery came from, but with the BMV installed this has ended that idea.
I was planning this on the new build too. I guess I'll have to treat it to a new 24 volt charger. Another thing that helps, but is rarely done is to swap the positions of the batteries.

 

[wildebus]

thinking about it, having the shunt on the 0V of the 24V batteries-in-series; the 12V chargers left as they are (one per 12V battery); and the secondary voltage input on the battery midpoint could be very useful. Ok, it may not tell you the total current in or out of the system while under mains charge, but it may be interesting to see how well matched the chargers are to each other, plus info on the batteries as it will tell you each batteries voltage (one via the midpoint reading, the other by voltage-midpoint) and how they vary to each other as the charging progresses.
In the perfect world, with identical batteries and identical chargers, the voltages will always be identical, but of course, nothing is ever actually identical.

 

[SquirrellCook]

This morning I checked the newly installed Victron BMV 712 on Murky's engine start batteries.
Not very good 24.04 volts this morning and 97% charge. The loading has not been more than 5 watts.
So looking at replacement batteries I looked at the installed Varta LFD90's I got from Battery megastore.
Low and behold they have a 4 year guarantee on them. So how long have they been fitted?
Shocked after trawling through emails only 18 months.
Now waiting for the response from Anita's email.

 

[SquirrellCook]

So yesterday evening when checking Murky's BMV's it was not good news. Although the engine start batteries were showing 97% the voltage had dropped just below 12 volts. The decision was made to charge them via the Victron inverter connected to the habitation batteries.
The two 24 volt 3.5 amps chargers were pulling 28 Ah's from the habitation. I didn't expect the load to be this high. Expecting the worse I went out for an hours exercise.
On my return I found the engine start batteries fully charged at 28 plus volts. The habitation was down the 77% at just under 12 volts on load.
Switching everything off the engine start batteries dropped to about 25.5 volts and the habitation rose to around 12.3 volts.
Early conclusions are that it's not the best way to charge the engine start batteries with limited solar to top up the habitation batteries.
Next time I will try again with more efficient chargers.

 

[wildebus]

Those starter batteries don't sound very healthy
A fast recharge time is just as suspect as a fast voltage drop.
The starter battery inheritated with my LT (just 12V system, not 24V though) was similar (but a lot worse) - would have a great voltage hold until tried to use the bugger, then it dropped bigtime - but it came back to "full charge" very fast. Swapped the battery out and not had a moments issue since.