Battery switching

[nomadros]

Hi...I've only posted here once before and to be honest, it didn't go well.
Anyway, I've a question regarding batteries.
I've 2 12V GEL batteries (they live indoors with me, hence the GEL purchase), different manufacturers and similar 230-250aH ratings.
My system is small, off grid and 12V.
I understand badness will occur if I parallel them and obviously I can't put them in series, so I was thinking...
Currently I use one battery for everything and I keep the second as a back up for bad weather days and keep that one ticking over with a battery charger (with GEL setting).
What I'd like to do is to have some sort of manual (or automatic) switching so that when the live battery hits a certain level I can swap over to the back up.

Does anyone know if such a thing exists and if so the pros and cons?

Currently what I do is switch off the inverter, go outside and disconnect the panels, then switch the battery cables, then reconnect everything back up but something better is needed.
I know the obvious answer is to just up everything to 24v and series the things but I've got around 480aH worth of batteries at 12V and a small PITA.
Any help appreciated.

[Ampster]

At marine hardware stores there are AB battery switches.

[Mike90250]

it's MUCH better for the batteries to simply run them in parallel, and only deplete the bank 40% (20% each battery), than it is to deplete one battery to 60% (40% low) THEN let it SIT IDLE, at low state of charge for a day or 3, while you are draining the 2nd battery down to 60%.
And you save all the switching back and forth.
But you can continue with your complicated plan if you want.

Are your batteries GEL or AGM ? Gelled electrolyte cannot accept a fast recharge, like solar. AGM batteries CAN be fast charged.

[GeorgeF]

Perhaps you can use a battery backup switching module like this one. Max output (load) 150W only.

[nomadros]

it's MUCH better for the batteries to simply run them in parallel, and only deplete the bank 40% (20% each battery), than it is to deplete one battery to 60% (40% low) THEN let it SIT IDLE, at low state of charge for a day or 3, while you are draining the 2nd battery down to 60%.
And you save all the switching back and forth.
But you can continue with your complicated plan if you want.

Are your batteries GEL or AGM ? Gelled electrolyte cannot accept a fast recharge, like solar. AGM batteries CAN be fast charged.

Apologies for the delay...they are GEL because they are in the same room as me. I bought the second battery because the first one went gaga when the outside temp went over 45c and I thought it was broken, so I bought another one (different make) so I could send the first one back to its supplier and still have power.

My daily use ends up at around 74-79% charge, however on cloudy days my fridge can take the battery down to 57%. The big problem with this is when the fridge compressor kicks in for a few seconds and can drop the charge down another 20%. A couple of times this has tripped my battery charge protector which is annoying.

I run one battery for a week at a time with the second on GEL trickle charge and switch only on 100% charge. I'm more than happy to parallel them up but mostly everyone says this is bad.

Off topic: I'm about to reform my house in the village. The wiring in the house is about the thickness of speaker cable, yet there is a big beat up old fridge from the 1960s in there. Between the wall socket and the fridge, there is a transformer which plugs into the wall socket and the fridge plugs into it. I was thinking "mmmm, that thing must be reducing the amount of power sucked through the speaker cable, might be useful." Does anyone know what this transformer is doing?

[Mike90250]

GEL & AGM are both sealed batteries, and both can vent explosive gas when overcharged or at end of life. Gel is not suitable for solar recharging, AGM is. Gel is silica gelled electrolyte, AGM is Absorbent Glass Mat. Read the battery label and figure out which you have.

You need to look at the box the fridge is plugged into, and see if there is a label on it. I'm not going to guess about a box with wires.

What causes an inverter to shut down - low voltage. Low voltage can be caused by either too thin of wires, too long of wires, or insufficient battery. You have to address each of those factors to resolve the problem.

[nomadros]

GEL & AGM are both sealed batteries, and both can vent explosive gas when overcharged or at end of life. Gel is not suitable for solar recharging, AGM is. Gel is silica gelled electrolyte, AGM is Absorbent Glass Mat. Read the battery label and figure out which you have.

You need to look at the box the fridge is plugged into, and see if there is a label on it. I'm not going to guess about a box with wires.

What causes an inverter to shut down - low voltage. Low voltage can be caused by either too thin of wires, too long of wires, or insufficient battery. You have to address each of those factors to resolve the problem.

Well I have 2 gel batteries, sorry. They are all advertised as GEL deep cycle batteries for solar power out here in Spain. The inverter/controller hybrid even has a GEL EURO setting.

The inverter shut down is by design when the battery gets too low and you can select cut out at 10.5v or 11v.. It resets when the battery gets back to 13v.
Case in point, yesterday, dark cloudy day, battery sitting at 74% all day but not recharging. Yesterday evening I decided to drink beer and watch much DMAX on TV. 2a.m. the fridge compressor kicked in for a second and I got woken up by the inverter beeping at me to warn me things were getting low, so I switched batteries and watched more DMAX.

Re the transformer, modern plugs won't fit it anyway, so no worries. The only label on the thing states 120-220v reversible 400VA..The house is 230v and the fridge has no labels. Amazing that this big fridge is still running after more than 50 years via some speaker cable and that the house hasn't burned to the ground.

Thanks for the reply though. This is my shakedown project to work out issues and learn.

[Xplode]

In your case, your batteries are mismatched and probably not overly expensive so unless your budget is extremely tight, I'd just either replace with a larger single battery, or hook these two in parallel until they're toast AND THEN buy a bigger battery. At that time, I'd revisit the rest of your setup for sure and make certain it is all correctly sized to work together (panels/controller/inverter/battery) as they really do require a little coordination to get the best value/success.

For a couple of relatively low cost 70Ah batts, I'd just hook em together instead of messing around and buying more bandaid fixes like an A-B battery switch.
Just my $0.02 though.

[nomadros]

In your case, your batteries are mismatched and probably not overly expensive so unless your budget is extremely tight, I'd just either replace with a larger single battery, or hook these two in parallel until they're toast AND THEN buy a bigger battery. At that time, I'd revisit the rest of your setup for sure and make certain it is all correctly sized to work together (panels/controller/inverter/battery) as they really do require a little coordination to get the best value/success.

For a couple of relatively low cost 70Ah batts, I'd just hook em together instead of messing around and buying more bandaid fixes like an A-B battery switch.
Just my $0.02 though.

Thanks for the reply and appreciate the advice... I'll probably parallel them as the cheapest option or given that I've 4 panels and only need 2, have a spare small 100W inverter in my truck and now 2 batteries, I might just buy another charge controller and have a low watt system for my lights, laptops etc and a second set up for the 1000W inverter for the fridge, blender etc.

As a side note, just a word on the Spanish mentality on solar panels. They don't like them.... because they've done a big investment in nuclear and they just love central control so they don't want people to be able to do things (like use electricity) without paying for it. Therefore solar panels are classed as "Urbanisme" (in Catalan) and you therefore need a building licence to install them in the countryside (where you can't build anything without a fight). I've been applying for a permit for about a year and the latest response is that I have to have a registered architect do the small project documentation and project control. This is just to mount 4 panels on a roof. If I want to ground mount or put them on a wall I need a second permit from the Department of Agriculture. However, I can have a diesel generator going bang, bang, bang and deafening the countryside no problem at all. My panels are currently leaning against a wall because I need the trailer they were mounted on for my harvests. Oh, try installing solar panels in town and hoo-boy world of hurt.. The conditions are so onerous you just give up.

[TKB4]

You might also find something like this useful in particular situations.

https://www.amazon.com/Bussmann-RB-BI-70A-Battery-Isolator/dp/B00OHGWC4A/ref=sr_1_4?keywords=battery+isolaters&qid=1571091899&sr=8-4