How to correctly wire 12v batteries

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23 posts

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#3033177 8-Feb-2023 06:41

"40A/20A is the maximum output of a 480W battery charger, calculated using Ohms Law.

480W /12V = 40A 480W /24 = 20A"

Ok thanks for this information, so what do you guys think about what output charger to use, 12/40amp, is 40amp a bit of overkill? as it's not like I'm draining the batteries all the time, is it possible to get by with a lower amperage.

frankv

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#3033216 8-Feb-2023 08:44

tonygeekzone:

Once you've chained all the negatives and all the positives, connect the charging inputs at opposite ends of each daisy chain.

One thing to be aware of is that the each link in the daisy chain will carry more current than the next. So, if you have 4 batteries in a chain and are putting 20A into them, the first link will carry 15A, then next 10A, and the last 5A. I think that was what they were trying to avoid in the original diagram.

Remember that you need to size the wiring to match the maximum current... if you have 20A current you need at least 2mm^2 (14AWG) wire. Undersized wiring will heat up, wasting energy and perhaps causing a fire.

trig42

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#3033229 8-Feb-2023 09:18

I think the first diagram you posted with the connections in the middle of the chain are just so that you don't need a bigger guage wire from the first to the second in the chain. Parallel is parallel, and so long as your wires are correctly sized as to not heat up, it won't matter too much. I'd run the negative off one end of the chain and positive off the other end as per your diagram.

What are you trying to achieve?

You say you'll have the charger permanently attached - are you building some sort of UPS?

Will your charger have some smarts so it doesn't overcharge the batteries, or let them go too low?

Do you already have the batteries? Have you looked at Lithium?

At our bach (off-grid) we had 3 x 105Ah 12V AGM batteries, connected to a solar array through an MPPT controller. It worked fine (running a 240V fridge through an inverter) for about 3 years, but we found that we were getting less and less out of the batteries. Instead of replacing Lead batteries, we demoted them to running lights/USB and bought two 24V 100Ah Lithium batteries and 600W panels (and a 24V inverter). We did the costings and the Lithium came out cheaper per usable Watt-Hour than buying more Lead Acid/AGM batteries (You'd need to buy nearly twice as many Amp-hours in lead as you should only use about 50% of their charge). No regrets and no battery-anxiety when it rains - the lithium batteries will run the fridge for about 2-3 days in the dark, and even a cloudy day on the 600W panels puts enough power in the batteries to run in overnight. The batteries have never gone below about 80% (according to their built in meters).

tonygeekzone

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#3033609 8-Feb-2023 17:51

frankv:
tonygeekzone:

Once you've chained all the negatives and all the positives, connect the charging inputs at opposite ends of each daisy chain.

One thing to be aware of is that the each link in the daisy chain will carry more current than the next. So, if you have 4 batteries in a chain and are putting 20A into them, the first link will carry 15A, then next 10A, and the last 5A. I think that was what they were trying to avoid in the original diagram.

Remember that you need to size the wiring to match the maximum current... if you have 20A current you need at least 2mm^2 (14AWG) wire. Undersized wiring will heat up, wasting energy and perhaps causing a fire.

Yes this is exactly what I want to avoid, which is why I came here, because so much conflicting information on the interweb, it does seem that unless I connect them in a very specific way, I'm going to run into this very problem of uneven charging, and yes I have the wiring understood, thanks.

tonygeekzone

23 posts

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#3033612 8-Feb-2023 18:10

trig42:
Parallel is parallel, and so long as your wires are correctly sized as to not heat up, it won't matter too much. I'd run the negative off one end of the chain and positive off the other end as per your diagram.

What are you trying to achieve?

You say you'll have the charger permanently attached - are you building some sort of UPS?

Will your charger have some smarts so it doesn't overcharge the batteries, or let them go too low?

Do you already have the batteries? Have you looked at Lithium?

At our bach (off-grid) we had 3 x 105Ah 12V AGM batteries, connected to a solar array through an MPPT controller. It worked fine (running a 240V fridge through an inverter) for about 3 years, but we found that we were getting less and less out of the batteries. Instead of replacing Lead batteries, we demoted them to running lights/USB and bought two 24V 100Ah Lithium batteries and 600W panels (and a 24V inverter). We did the costings and the Lithium came out cheaper per usable Watt-Hour than buying more Lead Acid/AGM batteries (You'd need to buy nearly twice as many Amp-hours in lead as you should only use about 50% of their charge). No regrets and no battery-anxiety when it rains - the lithium batteries will run the fridge for about 2-3 days in the dark, and even a cloudy day on the 600W panels puts enough power in the batteries to run in overnight. The batteries have never gone below about 80% (according to their built in meters).

thanks for your reply,yes doing a bit experimenting to see what is involved ups/off grid, yes I already have my batteries, they were left over from a camper conversion, and yes the charger will definitely need to be "smart" as I want it to come on/off automatically as needed, I'll be investing later in the year in a couple of 440watt panels for next summer, so I'll be running fridge, tv microwave, LED lights and a few other small low wattage devices.

neb

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#3033621 8-Feb-2023 18:24

elpenguino:
This should show the important thing you need to know. Once you've chained all the negatives and all the positives, connect the charging inputs at opposite ends of each daisy chain.

Is it worth mentioning adding some protection to that? If the OP dropped a screwdriver across any part of it it'd lead to some impressive fireworks.

tonygeekzone

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#3033633 8-Feb-2023 18:45

neb:
elpenguino:
This should show the important thing you need to know. Once you've chained all the negatives and all the positives, connect the charging inputs at opposite ends of each daisy chain.

Is it worth mentioning adding some protection to that? If the OP dropped a screwdriver across any part of it it'd lead to some impressive fireworks.

Absolutely I've been thinking about something just like this, some kind of boxed cover once completed, however while I'm actually connecting each one I'll be extra careful, all the connections will be bolted, I could maybe cover each completed set as I work along.

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neb

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#3033711 8-Feb-2023 20:42

tonygeekzone: Absolutely I've been thinking about something just like this, some kind of boxed cover once completed, however while I'm actually connecting each one I'll be extra careful, all the connections will be bolted, I could maybe cover each completed set as I work along.

I was thinking more along the lines of electronic protection (although the extreme case of a screwdriver dropped across the terminals is going to be difficult to protect against), thermal fuses, over/undervoltage protection, etc. Bing "cell monitoring unit", there's a lot of expertise and information online in this area around marine battery banks.

K8Toledo

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#3034186 9-Feb-2023 18:44

tonygeekzone: "40A/20A is the maximum output of a 480W battery charger, calculated using Ohms Law.

480W /12V = 40A 480W /24 = 20A"

Ok thanks for this information, so what do you guys think about what output charger to use, 12/40amp, is 40amp a bit of overkill? as it's not like I'm draining the batteries all the time, is it possible to get by with a lower amperage.

12V/40A is possibly overkill for charging batteries that are generally at least 80% full.

What's the total capacity again?

In our car is a 950 CCA 105 AH AGM behemoth. Recharging from flat (3V) to 12v (full) takes around 14hrs with 6A charger.

On a side note.....

I once tried to simultaneously charge two 12v 105AH batteries (same as above) using just my car and a set of jumper cables.

Not having a charger handy, I installed battery #1 in the car, jumped it then connected battery #2 to battery #1 in parallel with the jumper cables.

The cables melted. :D They were very cheap! lol

k1w1k1d

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#3034189 9-Feb-2023 18:54

Most of those car jumper leads are 90% plastic and 10% copper. Have seen lots of times where owner can't jump start their vehicle, but starts instantly when decent homemade cables are used.

EMB

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#3034242 9-Feb-2023 21:40

K8Toledo:

In our car is a 950 CCA 105 AH AGM behemoth. Recharging from flat (3V) to 12v (full) takes around 14hrs with 6A charger.

The maths in that doesn't really work - either the charger is doing more than 6A, the battery is less than 105AH, or it isn't fully charged after 14 hours.

105AH / 6A = 17.5 hours, plus about 10% to allow for the process not being 100% efficient, so a full charge is close to 20 hours. That said after 14 hours there will certainly be enough charge in the battery for it to do everything it needs to in terms of starting the vehicle.

K8Toledo

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#3034255 9-Feb-2023 23:18

EMB:

K8Toledo:

In our car is a 950 CCA 105 AH AGM behemoth. Recharging from flat (3V) to 12v (full) takes around 14hrs with 6A charger.

The maths in that doesn't really work - either the charger is doing more than 6A, the battery is less than 105AH, or it isn't fully charged after 14 hours.

105AH / 6A = 17.5 hours, plus about 10% to allow for the process not being 100% efficient, so a full charge is close to 20 hours. That said after 14 hours there will certainly be enough charge in the battery for it to do everything it needs to in terms of starting the vehicle.

The battery is def 105AH but I pulled 14hrs OTTOMH.

In all likelihood it would have taken closer to 20 hrs to fully charge as you said.

Incidentally I went through two chargers before the 6A unit.

One, a cheap 2.5A Projecta could not charge if battery voltage dropped below 9V. It keep switching on and off, prob due to OCP kicking in.

The other, a 4A charger blew up.

The 6A was a 1970's era unit given to me by a friend.

Similar to this:

tonygeekzone

23 posts

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#3034273 10-Feb-2023 06:41

K8Toledo:
tonygeekzone: "40A/20A is the maximum output of a 480W battery charger, calculated using Ohms Law.

480W /12V = 40A 480W /24 = 20A"

Ok thanks for this information, so what do you guys think about what output charger to use, 12/40amp, is 40amp a bit of overkill? as it's not like I'm draining the batteries all the time, is it possible to get by with a lower amperage.

12V/40A is possibly overkill for charging batteries that are generally at least 80% full.

What's the total capacity again?

In our car is a 950 CCA 105 AH AGM behemoth. Recharging from flat (3V) to 12v (full) takes around 14hrs with 6A charger.

On a side note.....

I once tried to simultaneously charge two 12v 105AH batteries (same as above) using just my car and a set of jumper cables.

Not having a charger handy, I installed battery #1 in the car, jumped it then connected battery #2 to battery #1 in parallel with the jumper cables.

The cables melted. :D They were very cheap! lol

Total is 440Ah, I'm not sure exactly how much draw I'll be using, but it's not much to begin with.

Ge0rge

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#3034274 10-Feb-2023 06:44

@K8Toledo - why are you allowing your battery to get down as low as that? A 12v AGM is essentially flat at around 10.50v. Taking it below that (and to be fair even as low as that) is just doing needless damage and significantly shortening its useful life.

Perhaps it's a typo, but I just can't see how it's of any use to you at 9v, let alone 3.

elpenguino

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#3034336 10-Feb-2023 09:36

neb:
elpenguino:

This should show the important thing you need to know. Once you've chained all the negatives and all the positives, connect the charging inputs at opposite ends of each daisy chain.

Is it worth mentioning adding some protection to that? If the OP dropped a screwdriver across any part of it it'd lead to some impressive fireworks.

It is worth mentioning. Each battery should be fused but the fuse needs to be very close to the positive terminal because the fuse is there to protect the cabling. The fuse will have to be higher rated than the expected load to prevent nuisance tripping.

Also, for an unattended installation, is there some kind of low voltage disconnect to stop the batteries from being drained if/when the charging system doesn't work?

Depending on the battery chemistry, is there a risk of hydrogen gas produced during charging ? no sparks/confined spaces etc.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

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