Balancing Batteries / Mixing Types in Battery and Solar - Problems, Issues & Solutions ?

RAW

Full Member
I am not that well off financially.....
So what is best practice in saying mixing Leisure Batteries, if I wanted to Mix 160AH AGM and 110AH Sealed LA then how best to go about it. Also I know it is not best to Mix Solar Panels, but if I was adding to an existing Solar set-up what would be best practice ?

I have some thoughts myself, like with different batteries maybe use a Battery Balancer ?
With Solar maybe try and match output of current panel with that of an additional panel ?
Any input appreciated, TIA
Robert
 

wildebus

Full Member
Those Battery Balancers are designed to keep a pair (or more) 12V batteries balanced to each other when connected in Series. You would not use them on 12V batteries connected in parallel, and they would tend to balance themselves anyway (a bit like two water containers connected together - fill one container up and water will flow from one to the other to equalize the levels by themselves. For Water, read Voltage).
Is this second SLA battery the one you won on eBay? If so, it is a bit of an unknown quantity and I would be inclined to treat it as a 'secondary' battery, just connected to the 12V system and with some kind of mechanical switch or basic 12V relay to bring in and and out of the system (similar to the Starter Battery, where that becomes part of the system when the Split-charge relay is activated).
Downside is the BMV SOC information won't be accurate with this setup, even if the 110Ah battery is on the BAT side of the BMV shunt, but I would be wary of having the two together permanently. You want to be able to charge the 'proper' AGM battery up 100% whenever possible and not have that mucked up in some way by the unknown one.
I wouldn't go for having two seperate solar setups, not much point and won't get the most out of them that way.

(so above is a long-winded way to say not ideal to mix them!)
 

Nabsim

Full Member
If I wanted to do this I would just fit a cable between both banks with a battery isolator switch so when I want to charge the LA I turn switch on and let them equalise. I think that would work but don’t mine being corrected if not 👍

I would treat whatever I powered from the LA as separate though, such as for charging phones, tablets and emergency backup. you could put a cheap voltmeter on the battery to show approximate SOC.
 
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RAW

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with some kind of mechanical switch or basic 12V relay to bring in and and out of the system (similar to the Starter Battery, where that becomes part of the system when the Split-charge relay is activated).
Downside is the BMV SOC information won't be accurate with this setup, even if the 110Ah battery is on the BAT side of the BMV shunt, but I would be wary of having the two together permanently. You want to be able to charge the 'proper' AGM battery up 100% whenever possible and not have that mucked up in some way by the unknown one.
I wouldn't go for having two seperate solar setups, not much point and won't get the most out of them that way.
Thanks, that's pretty much what I have on Sylvia now so will do something similar I guess, and to answer your question, I may soon buy another van, and it will have a Leisure Battery which if in a worse state or lower AH than the 110AH one that I won on EBAY will go back into Sylvia.
Cheers @wildebus and @Nabsim
 

Nabsim

Full Member
I have only used same panels together so don’t know. I know it can depend on panel voltage and controller but Dave is the expert here.
 
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SquirrellCook

Full Member
With panels I suspect if they were in series, the lowest current panel would limit the others.
Not so sure in parallel as I guess the stronger panel would drive the volts up, but I expect the weaker would be heated.
Kirchhoff type problem me thinks.
 

Nabsim

Full Member
To be honest if I have electrical queries I just ask Dave. I thought I knew a bit about 12v electrics from years of bikes and messing with cars but had o think again when it came to the van lol so much I hadn’t come across before.
I can’t afford the stuff Dave would use normally but his advice is always good 👍
 

wildebus

Full Member
The altE Solar store has some good info regarding this question as well as a demonstration of the effect of mixing panels and I tend to believe their testing a lot more than other resources (and they have the weather to verify this kind of stuff :) )
I have copy/pasted some of the points in their explanation which can be found here - https://www.altestore.com/blog/2012/08/adding-capacity-to-your-existing-pv-system/ and embolded the key points, as well as a few extra things to note.

1) IN SERIES
Panel voltage is additive when panels are wired in series. The only way to wire in additional panels and guarantee that the voltage of all strings would remain equal is to add the exact same number of new panels to each string. The drawback to doing this however is that within each string all panels will produce the same current as the current of the lowest panel. So if you buy a panel that has a higher current it won’t produce the amount of current that is listed on its nameplate (it would perform at the lowest performing panel’s current), and you’d essentially be paying for a panel that will produce less power than its rating.
Note that this means if you bought a panel with a LOWER current spec of the existing panel/s you would be downgrading all the existing panels.
You would only have multiple panels in series if you had an MPPT controller (see the third bullet). PWM Controllers essentially truncate the 'excess' voltage so add a second panel in series with PWM ... you will get no extra power and could in fact reduce the arrays output.



2) IN PARALLEL
Panel current is additive when panels, or strings of panels are wired in parallel. Say you were to try to just add the new panels all in one or more strings composed of only the new panel types. While each of these strings would be able to produce the maximum amount of current they are rated for, the voltage difference between the old strings and new strings becomes a problem. If the panels in the new strings had a higher voltage rating than the panels in the old strings, the new string, and each panel within it, will only be limited to operating at the same voltage as the old strings. So again, you’re essentially paying for a panel that will produce less power than its rating.
If you are using a PWM controller, then high voltages are wasted anyway. If you are using an MPPT controller then the above bold point applies. But also note the third bullet below.


3) USING MPPT CONTROLLERS
If you are using an MPPT charge controller, mixing panels of different voltages and/or currents within an array will have an adverse effect on the entire system. Different module outputs make it impossible for the controller to determine the optimal operating voltage and current because they will be different between the different types of panels.
What this point is basically saying is that the main feature of MPPT - that is the Maximum Power Point Tracking algorithm - gets confused and can no longer work optimally. So adding a random 100W panel for example may not only not give you the full 100W due to points 1) and 2) above, but also stop the whole original setup working as well as before.


So my own summary would be:
Try and get the same panel specs (matching Voc and Isc at the basic level) (Here is another link from altE with a good guide - https://www.altestore.com/blog/2016/04/how-do-i-read-specifications-of-my-solar-panel/ )
If you have a PWM controller, add in parallel but don't waste money on getting anything other than "12V" (so ~20V Voc) Panels
If you have MPPT, avoid mixing and if you do so, be prepared for a level of disappointment
If you do have to fit mismatched panels, consider multiple controllers - one for each standard.



Finally, to get round the problem of mismatching, you can get cable them a little 'creatively' to make them match more :) Last link, again with the altEstore which talks about this ....
 

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