Charging 12v batteries in parallel

[jambouk]

If I have one solar panel, and four 12v batteries I need to be charged, two pairs of equal age and capacity (60 amp hour x 2, and 50 amp hour x 2) and the pairs have the same resting voltage and seemingly the same capacity, and I can get a solar charger unit which can charge two batteries (see link below), I was thinking of charging each pair in parallel.

I've read about the risks of charging in parallel, with the weaker battery having the dominant impact and bringing the other battery down to its capacity, but in reality, how likely is this to be a problem in the course of 5 years, where the batteries won't be used that much anyway, maybe discharged 20%, just 10 times a year? 

I'm conscious there is scope to discuss lots of things around this, like "buy pairs of new batteries rather than starting out with old ones" and "buy larger capacity new batteries etc", but this is the set up we have and will be sticking with.

Thanks for any replies.

James

https://uk.rs-online.com/web/p/solar-power-regulators/9054536?cm_mmc=UK-PLA-DS3A-_-google-_-PLA_UK_EN_Power_Supplies_And_Transformers-_-Renewable_Energy|Solar_Power_Regulators-_-PRODUCT_GROUP&matchtype=&pla-391749164546&s_kwcid=AL!7457!3!243856857002!!!g!391749164546!&gclid=CjwKCAiAu9vwBRAEEiwAzvjq-zrEd9pZ-QRkhK-Tpfd6UJFYhRnC8sQOzas0oXPQXq3zr44tGo3_MhoC_XcQAvD_BwE&gclsrc=aw.ds

[Dr_Ju_ju]

Personally, I'd build a couple of these, https://www.electronicshub.org/lead-acid-battery-charger-circuit/ One for each battery, with a common source, whether that be a DC (Solar Panel if it's able to supply the volts under load), or an AC supply....

 

[Alien 13]

I am wondering if fitting  a 20A diode in series with each of the batteries would work, there will be some voltage drop but it should still be enough to charge them while keeping them partially isolated.

Alan

[jambouk]

52 minutes ago, Dr_Ju_ju said:

Personally, I'd build a couple of these, https://www.electronicshub.org/lead-acid-battery-charger-circuit/ One for each battery, with a common source, whether that be a DC (Solar Panel if it's able to supply the volts under load), or an AC supply....

 

Is the link suggesting this is just for batteries with capacity of 1-7 amp hours?

 

[Carbon Brush]

The first questions to ask.....

Are the batteries going to be powering separate loads?

Are the batteries going to be permanently connected to the chargers(s)? Even when driving a load.

David.

[Gina]

I don't think that circuit is suitable for the sort of batteries you would use for astronomy.  You'd probably want to charge around 5A as with a car battery charger.  Some years ago I designed and built a 5/10A battery charger for our tractor battery.  I guess solar panels may only be capable of trickle charging though unless big.  Have to say I'm not very familiar with solar panels.

[symmetal]

If the batteries are permanently in parallel that is fine. Diesel-electric submarines have loads of lead-acid batteries in parallel. If not permanently connected together then a diode in series with each battery is recommended to stop high currents (and sparks) if one battery is much flatter than the other when you connect them together for charging. Use a schottky diode as these have a lower forward voltage drop. Ideally you need to increase the float charging voltage from the charging controller to compensate for the diode drop.

Alan

[Skipper Billy]

All our boats have at least 3 batteries in parallel - charge together and discharge together - no problem.

[jambouk]

The batteries will be kept in parallel all the time, and will be connected to the charging unit all the time, I'd envisaged even whilst under load, which will most likely be at night when the solar panels are not producing, but the batteries may also be put under load during the day time when the solar panels are active. One pair of batteries will power one thing, and the other pair power something else.

[jambouk]

4 minutes ago, Skipper Billy said:

All our boats have at least 3 batteries in parallel - charge together and discharge together - no problem.

Thanks. What is the capacity of each battery, and how long have you been using this set up with the same set of batteries?

James

 

[symmetal]

2 minutes ago, jambouk said:

The batteries will be kept in parallel all the time, and will be connected to the charging unit all the time, I'd envisaged even whilst under load, which will most likely be at night when the solar panels are not producing, but the batteries may also be put under load during the day time when the solar panels are active. One pair of batteries will power one thing, and the other pair power something else.

That should be no problem. The series diodes aren't necessary.

Alan

[DaveL59]

from reading I did some time ago, best to normalise the batteries before hooking them together, so there's minimal voltage difference in which case you shouldn't get any arcs/sparks. Are you gonna use a controller between the panel and batteries?

[jambouk]

Something like this:

 

https://www.amazon.co.uk/ECO-WORTHY-Controller-Auto-adaptive-Digital-Display/dp/B01B4AK9SM/ref=sr_1_10?keywords=Dual+Battery+Solar+Charge&qid=1578578415&sr=8-10

 

[symmetal]

I like the description on the Amazon site James posted above which describes it as having a dual LED nixie tube display. I'm sure there aren't many people under 50 who know what a nixie tube is.  7 segment LEDs replaced nixie tubes in the 1970s.

Alan

[Davey-T]

RS price looks rather exorbitant, I fitted identical one in my camper van that cost less than a tenner.

Dave

[jambouk]

10 minutes ago, Davey-T said:

RS price looks rather exorbitant, I fitted identical one in my camper van that cost less than a tenner.

Dave

I’ve since found the Amazon one on Ebay for £14. 

[theropod]

Having lived off the grid for 20 years, and using a bank of multiple 12V lead-acid batteries I can attest that operating such batteries in parallel is common practice. We currently have 16 6V golf cart batteries wired into 8 pairs of 12V batteries providing somewhere north of 1400 amp hours of capacity. Fusing is more critical than diodes, which are power wasting little devils. A 50 amp breaker protects each positive lead of a 12V pair.

[jambouk]

3 hours ago, theropod said:

Having lived off the grid for 20 years, and using a bank of multiple 12V lead-acid batteries I can attest that operating such batteries in parallel is common practice. We currently have 16 6V golf cart batteries wired into 8 pairs of 12V batteries providing somewhere north of 1400 amp hours of capacity. Fusing is more critical than diodes, which are power wasting little devils. A 50 amp breaker protects each positive lead of a 12V pair.

Thanks, that is reassuring. Are you placing a fuse on the positive lead between the controller and the pair of batteries, or between the batteries and the thing they are powering? Or both?

James

[jambouk]

Should I put fuses where the green boxes are, or the purple boxes?

[theropod]

8 hours ago, jambouk said:

Thanks, that is reassuring. Are you placing a fuse on the positive lead between the controller and the pair of batteries, or between the batteries and the thing they are powering? Or both?

James

I just put the breakers on the positive lead of each 12V pair, which all the positive leads and negative leads attach to positive and negative 1/2” x 1” X 42” solid copper bus bars. Those bus bars reside behind the batteries in a divided compartment of the box, and a 3” plumbing fitting allows cable access to the outside world. The inverter/ charger, solar charge control and wind (aside from a dump load for the wind) all are fed via these bus bars. All the cable, batteries and bus bars live in a locking thick steel decommissioned Union Pacific railroad crossing control box, and this is all positively ventilated to get rid of hydrogen gas produced from charging the lead/acid batteries. The inverter and solar charge controller live outside that box in their own double dry screened in box about the size of an apartment size fridge. The chances of water intrusion into either box is nil.

No bugs or rodents have ever caused any issues with the electrical system itself, but yellow and black paper wasps have found the rain drip rail over the top of that steel box to be “just right” for them to set up shop. That box sits under the protection of a small shed, and never sees full sun. I have to deal with at least one new nest of the beasties every spring.

TMI?