Lithium Batteries in Parallel Unequal Discharge

[LeeME3]

I have 2 Renogy 100Ah lithium batteries wired in parallel to bus bars. They work exactly as I'd expect and give slightly better performance than anticipated. However, one ALWAYS discharges a bit quicker than the other even though they are wired directly together and the bus connections go to the + terminal of one and the - terminal of the other. Any ideas what it could be? Even if one battery is weaker surely they should still equalize even if it is a case of the weaker one pulling the stronger one down. They have built in bluetooth and BMS but are not the ones with the ethernet interconnect. It is as if the BMS are fighting each other. I've raised a case with Renogy but any thoughts most welcome!

[Rosie n' Jim]

I'm confused by this. Since both batteries are connected pos to pos and neg to neg, they have to be at the same voltage, unless one of the cables has a high resistance, most unlikely! Are you measuring the battery voltage with a meter of trusting the bms bluetooth data?

[LeeME3]

I'm confused too as my logic is the same as yours! It is quite difficult to measure by voltage as, being litihum, they hold their voltage really well until below 50% or even lower. So, I am largely reliant on the bluetooth connection from the BMS to the Renogy app. However, since then I've installed a shunt based meter (the Renogy one, see pic) and use that instead (I sort of force myself to not look at the app!). Of course that just measures the 2 batteries as one large one but it stops my paranoia! My next job is to put some isolator switches in (the big red key types) so I can select one, the other or both batteries to be in the system - then the shunt meter will tell me more accurately what is going on.

In all honesty on a recent 3 day trip with the fridge running and the 175w solar panel working nicely I never dropped below 90% total capacity (180Ah) so I'm trying to not think about it too much! It may just be a case of 'too much info' and me overthinking it.

[Rosie n' Jim]

I agree that you may be over thinking this. It's too easy to become paranoid with these old vehicles, I Di with the temperature on my Aircooled T25'so. 
As you say, lithium battery voltage doesn't drop much, then falls off a cliff! I'm a firm Renogy fan, so feel the meter will give a good idea of what's going on. I don't think individual battery isolators will help you , more hassle that it's worth. 
I feed my fridge from the output of my Renogy  40amp controller. It shows the Ah in and out, but probably not as accurate as a shunt meter. I use a Halfords compressor fridge and 110Ah lead acid battery with 200w panels mounted flat on the roof, not very efficient, but the battery is full by about 9am on a bright day. Never seen the battery below 80% off load, and the battery is really rather old.

[LeeME3]

Yes I like the Renogy stuff. After years of using a variety kit and brands I've settled on Renogy and Victron. The shunt meters are on offer at the moment and it was dead easy to fit and set up. I've also got a Renogy DC to DC charger and am about to swap the Victron solar controller to the Renogy one (again as it was on offer recently). Not because I have any issues with the Victron one but the Renogy one is a bit bigger and there is room for it in the van which will free the Victron up to go in the Eriba Puck in which space is rather more limited! I have a Renogy 1000W inverter too and, unlike most inverters, it easily meets the spec it claims even for continuous output.