Charging batteries

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phoenox

New Member
The other day I used a 12 V house battery to run a 120 V AC inverter to power a battery charger to charge a 12 V tractor battery.

I am thinking it would be way simpler (and more efficient) to just hook the two batteries up directly, but I hesitate to do this because I do not know if it will cause some damage to the batteries if they are hooked up while one is charged and the other discharged. The house battery is part of a solar power system and quite expensive, I do not want to take any risks with it. Maybe I am just being overcautious?

I have considered using a constant current supply that will run on 12 V but this may be making things more complicated than they need to be. Can anyone recomend a safe, simple way to charge one battery from the other without converting to 120 V then back to 12 again?

tcmtech

Banned
Jumper cables!

They work great!

Been around as long as batteries too!

crutschow

Well-Known Member
I think your concern about charging current is valid. The charging current into the discharged battery could be quite high and perhaps damage the battery if it is not limited. Jumper cables are ok if they are only used for a short time so battery damage is not likely. For your application a current limiter is desirable, as you suggested.

If you want to go minimum, you could just use a power resistor. Since the voltage difference between a discharged (say 25% charge) and a fully float charged battery is no more than about 2.5V, a 1Ω, 10W resistor would limit the current to 2.5A. The current would taper off as the battery charged.

If you want a higher charging current you could use a smaller resistor, but the peak resistor power will go up (P = 6.25 ÷ R).

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tcmtech

Banned
You guys must not jump start stuff!

I have jump charged 6 volt batteries from 12 volts countless times too!
and 12 volts from 24 volts as well. Both are common farm and construction practices.

I have never seen a good battery damaged by charging off of another battery.
I am rather sure its been done a few billion times in the last 100 years without much problem. Unless your stupid enough to connect the batteries in the wrong polarity!

This is just one more example of how real life works just fine even though theoretical says different.

It can be argued against in countless ways but in all reality its a common thing so dont worry to much.

You can always pay AAA to come and do it for you and if asked I bet he says he does it thousands of times a year and will look at you like your retarded when you point out your charging current concerns!

Chippie

Member
I think your concern about charging current is valid. The charging current into the discharged battery could be quite high and perhaps damage the battery if it is not limited.

If you want a higher charging current you could use a smaller resistor, but the peak resistor power will go up (P = 6.25 ÷ R).

Charge current into a discharged battery is limited by the battery's own internal resistance....

What type of battery is used in the 'solar system'?
Is it a sealed lead acid gel type?

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crutschow

Well-Known Member
As I pointed out, the short time you connect jumper cables is not likely to damage a battery. I was concerned about long term connection of a partially charged battery, which has a very low internal resistance, connected for a long period of time to a fully charged battery. Perhaps it's not a problem, but I'd rather be on the safe side. It's your call.

Willbe

New Member
I have jump charged 6 volt batteries from 12 volts countless times too!
and 12 volts from 24 volts as well. Both are common farm and construction practices.

I have never seen a good battery damaged by charging off of another battery.
You might have shortened its life.

Let's say you did it 500 times and the chance of the battery exploding with this treatment is 1 in 1000.
Your chance of getting away with doing this successfully 500 consecutive times is about 100[1-{0.999^500}] = 40%.

I guess I'd wear face protection in any case.

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Chippie

Member
You guys must not jump start stuff!

I have jump charged 6 volt batteries from 12 volts countless times too!
and 12 volts from 24 volts as well. Both are common farm and construction practices.

concerns!

So, you got away with it so far..

I dont recommend you continue with this practice....

Banned
hmm, lets do some math here. Random website says a fully charged LA has an internal resistance of .01Ω and a voltage lets say around 12.6 volts
I don't know what the resistance is when the battery is discharged but lets say it's 10 times it's charge value, and the voltage is 10 volts.
That's 2.6 volts and .11Ω or about 26 amps of current, sounds not too unreasonable to me. Thing is it won't come even close to fully charging the discharged battery because as the batteries equalize the current will drop to nothing. Best you from a fully charged to a dead battery is probably about 25-40% charge. Just don't quote me on those internal resistances because if I'm off but .1 ohms and it's actually .01 ohms total the current is gonna be close to 300 amps... that's not so good. Fine for jumping not for charging.

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tcmtech

Banned
I have done jump starts to batteries the were at the point just below turning the starter over. I have done it before with a constant voltage welder generator set at 15 volts and with an amp capacity of over 600 amps peak.
The 12 volt batteries will pull around 200 amps for about 5 seconds and then rapidly tapper off to around 30 - 50 amps within a minute or so and continue to hold that rate for some time after that.
Normaly the battery is just momentarily subjected to that high rate of charge just long enough to get the engine to start. Then the charging system takes over from there.

Shortening a $100 battery's life doesn't really matter when the machine its in costs around$80 an hour on site. More if it causes other equipment and people to have to stand around while it charges up so it can start.

But I dont think a short burst charging at high amps has any effect though. Its not any harder on a battery than the high amp draw it gets when starting something.
My understanding is that when a battery is force charged until it heats up and starts to cook internally thats when the life gets shortened.

I take safety precautions when jumping batteries with higher voltages. but I have had a few batteries blow up while on a 10 amp charger too though.
Rather makes a person concerned when you hear a shotgun like bang come out of you shop when you in the house!

phoenox

New Member
The house battery is a lead acid battery, but not sealed. It is made of 6 smaller batteries conected in series. Measures about 5' long X 3' high by 1' wide. This battery cost well over \$1000.

I think I will get myself a power resistor that I can use to trickle charge other batteries. Although this is still not terribly eficient, it is simple and safe.

Banned
Just don't try it with Lithium Batteries.

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