Bit thick about batteries

[Sharon the Cat]

We have a solar panel and 2 x 110ah leisure batteries.
Our control panel displays the battery voltage. I plugged in my little 12v blanket in when it was cold in the Cairngorms the other day and could see it was drawing 4 amps. No data signal so I couldn't Google any formulas to see how long I could use it for and unplugged it again.
I'd like to be a little more knowledgeable about our leisure batteries capability/capacity
I'm thinking 13.v something is good, but what is bad, or approaching bad and how do you work out how much juice you have for something hungry?

 

[Derekoak]

To an extent it depends on whether your batteries are flooded lead acid or Agm or some other improved chemistry but assuming flooded lead. 13v I think is due to charging from the solar. Something like 12.7v is a full voltage but as soon as you charge or discharge it changes, so you only can really tell how much juice you have if you read the voltage when neither charging or discharging has happened in the last say half hour. The deeper you discharge lead the more quickly it wears out. People say maximum discharge is 50% so your 2 x 110 should only ever be used for 110amp hours from totally full to stop. So your 4 amp blanket could be used for 27 hours theoretically but the faster you discharge the less efficient the discharge ,your batteries are not new I assume so deteriorated and even 50% discharge will cause significant damage. Try to keep to 25% depth of discharge and recharge to full as soon as suitable.

 

[Fisherman]

Well first it’s important to note what type of batteries you have, are they normal or are the AGM, or possibly even lead carbon.
AGM batteries are more durable, they charge better, are maintenance free, and they tend to last longer.
Batteries don’t take to well to rapid discharge, they respond better to a slower rate of discharge.
You say that you have two 110ah batteries well that gives your a max usable load of 110amps, and that’s the max before damaging or shortening the lifespan of your batteries. Now your 12v 4amp blanket over an 8 hour sleep will have used 32 precious amps.
Lead acid batteries lose voltage when discharging, it’s this voltage that indicates how much load is left for use. For a standard battery it’s fully charged at 12.7 volts. It reaches 50% discharge at 12.2 volts. For AGM the figures are slightly different. Now going below 12.2 volts is not good for your batteries. In winter I try never to go below 12.3v if possible, but I have went down to 12.2v twice.
This chart may help. But it may not be accurate for your particular battery. You may be able to google for more accurate figures from the manufacturer.

CAPACITY IN %	LEAD-BATTERY	LITHIUM BATTERY	LITHIUM AV-BATTERY
100%	12.70 V.	13.60 V.	12.60 V.
90%	12.50 V.	13.32 V.	12.10 V.
80%	12.42 V.	13.28 V.	11.60 V.
70%	12.32 V.	13.20 V.	11.35 V.
60%	12.20 V.	13.16 V.	11.10 V.
50%	12.06 V.	13.13 V.	10.80 V.
40%	11.90 V.	13.10 V.	10.70 V.
30%	11.75 V.	13.00 V.	10.60 V.
20%	11.58 V.	12.90 V.	10.45 V.
10%	11.31 V.	12.00 V.	10.25 V.
0%	10.50 V.	10.00 V.	9.00 V.

 

[Derekoak]

Someone will send a link to a state of charge versus voltage table. But you can only really use these values if you have the "neither discharged or charged voltage" as I said earlier. You may find it hard to get that value as your battery may nearly always be doing something.

 

[Deleted member 56601]

Battery monitor is the only accurate way. Pricey but worth it IMO. Nasa and Victron are 2 popular makes.

 

[Fisherman]

I find this gizmo very useful. It is simply connected to a battery then stuck onto the outside. It gives accurate voltage, and temperature. It also sends the information to your victron controller, optimising charge taking temperature into consideration. It has Bluetooth and is easy to use.

Victron Smart Battery Sense Long Range 10-meters MPPT Solar Charger SBS050150200 | eBay

Victron Energy. Victron Energy SBS050150200. The Smart Network is set up using the Victron Connect app. SmartSolar MPPT RS. Solar Panels. Fuses & Fuse Holders. Solar Panel Kits. Smart Network - a wireless technology based on Bluetooth Smart.

www.ebay.co.uk

 

[st3v3]

As above, voltage is useless for trying to work out how much charge is left in a battery. It would only be relevant if you disconnected the battery and left it idle for some time, then measured the voltage.

 

[Deleted member 56601]

I find this gizmo very useful. It is simply connected to a battery then stuck onto the outside. It gives accurate voltage, and temperature. It also sends the information to your victron controller, optimising charge taking temperature into consideration. It has Bluetooth and is easy to use.

Victron Smart Battery Sense Long Range 10-meters MPPT Solar Charger SBS050150200 | eBay

Victron Energy. Victron Energy SBS050150200. The Smart Network is set up using the Victron Connect app. SmartSolar MPPT RS. Solar Panels. Fuses & Fuse Holders. Solar Panel Kits. Smart Network - a wireless technology based on Bluetooth Smart.

www.ebay.co.uk

I think battery sense is for temperature only, not voltage. The information is used by the MPPT controller to adjust the charge rate

 

[Fisherman]

I think battery sense is for temperature only, not voltage. The information is used by the MPPT controller to adjust the charge rate

No it gives voltage also, and as it comes straight from the battery it’s more accurate. Right now my panel reads 13.6v, my MPPT 14.01v, but the battery monitor gives 14.06v. And it’s temperature is 9c. Here’s a screenshot Chris.
I bought the model with reduced Bluetooth range of 3m. For £28.

 

[Derekoak]

I used to find a voltmeter accurate enough as long as I woke up before dawn as at that time , no solar and our fridge was off overnight and nearly everything else except usb adaptor leds.
My new system will have a battery (monitor) management system and a shunt battery monitor as it will be lithium. Probably overkill.

 

[Sharon the Cat]

Thank you to all. I should have known it wouldn't be simple. I think I have a better understanding now though.

I'll check our batteries when we get home at the weekend.

 

[BigAldo]

I use one of these. Not too expensive and fairly easy to fit:
12 volt battery charge monitor

 

[Phantom]

I use one of these. Not too expensive and fairly easy to fit:
12 volt battery charge monitor

Me too, they work very well.

 

[trevskoda]

For those batteries you should have at least 200w of solar.

 

[Robmac]

For those batteries you should have at least 200w of solar.

I get by easily with 100W of solar on my 2 Trojans Trev - including running a compressor fridge.

 

[trevskoda]

I get by easily with 100W of solar on my 2 Trojans Trev - including running a compressor fridge.

Not in mid winter rob, maybe high summer, i get about 1 amp from both which is just about able to keep the bats from discharging when the van is idle.

 

[Robmac]

Not in mid winter rob, maybe high summer, i get about 1 amp from both which is just about able to keep the bats from discharging when the van is idle.

Depends on how you use your van Trev. I've never drained my batteries either in Summer or Winter.

 

[trevskoda]

Depends on how you use your van Trev. I've never drained my batteries either in Summer or Winter.

Yes i dont drain mine down but dont require much of a mains charge except maybe a few weeks in deep winter.

 

[Robmac]

Yes i dont drain mine down but dont require much of a mains charge except maybe a few weeks in deep winter.

I never mains charge my leisure batteries Trev.

I ran for 2 years without any solar at all with no problems, but now my charge controller/alternator/solar panel give more than enough.

 

[barryd]

Ive been studying batteries a bit since I thought mine was bust but it turned out I was just playing Smash Karts and watching youtube too much probably and it seems a lot of us might have the wrong sort of charger for these AGM Batteries which seem one of the better options these days. I was going to order a 130ah AGM battery designed for off grid use from Alpha but then I started reading their very useful guides and with an AGM your charger needs to be able to charge up to 14.6-14.8v. Most chargers pre 2017 only charged up to 13.8-14.4 it says and that kind of fits in with what my current 240v charger and MPPT controller deliver.

So if you bought an AGM battery and did not have the right charger you will only get up to 80% capacity out of it and it will remember that and never go above. So it says. My current Alphaline is sealed calcium I think and 125ah but I reckon its the same as it was never as good as my original lead acid 110ah banner.

My thinking when I get home is an AGM battery and a new 240v charger but then I may also need to change my MPPT Controller.

A Detailed Guide to AGM Batteries - Alpha Batteries

With AGM products becoming more widespread in their use, knowing when you can or should use one and how to look after it properly is all the more important - our guide is here to help.

www.alpha-batteries.co.uk