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A question about charging one battery with another


SilverNodashi
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Hi, 

 

Ok, so it is generally advisable not to mix battery types and sizes, right? But, what effective way is there to charge, say, an AGM battery from a deep cycle battery? 

 

Scenario: In my 4x4 I have a normal "car battery", rated at 105Ah. And then I have a 102Ah AGM battery to use for the fridge, lights, cellphone chargers, etc. There's a "dual battery system" installed, with a relay and (I think?) a charge regulator in between the two batteries. The system works well. 

 

My question is: would this not be bad for the two batteries? I don't think the AGM battery would have enough "cranking Amps" to start the 2.8 Turbo Diesel engine, which is why there's a car battery as well. 

 

Any thoughts?

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Hi, 

 

Ok, so it is generally advisable not to mix battery types and sizes, right? But, what effective way is there to charge, say, an AGM battery from a deep cycle battery? 

 

Scenario: In my 4x4 I have a normal "car battery", rated at 105Ah. And then I have a 102Ah AGM battery to use for the fridge, lights, cellphone chargers, etc. There's a "dual battery system" installed, with a relay and (I think?) a charge regulator in between the two batteries. The system works well. 

 

My question is: would this not be bad for the two batteries? I don't think the AGM battery would have enough "cranking Amps" to start the 2.8 Turbo Diesel engine, which is why there's a car battery as well. 

 

Any thoughts?

The relay closes when the engine is running and the alternator charges both batteries, the relay opens again when the engine is switched off, preventing the battery (fridges etc) from draining the 4x4's battery.

Some installations have diodes to prevent cross charging.

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The relay closes when the engine is running and the alternator charges both batteries, the relay opens again when the engine is switched off, preventing the battery (fridges etc) from draining the 4x4's battery.

Some installations have diodes to prevent cross charging.

please explain the relays a bit more?

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The relay is energised by the alternator. So the car starts off just the one battery, and as the rpms ramp up and the alternator comes online the relay is energised and it closes the connection between the two batteries. The two batteries now charge from the alternator. When you turn the vehicle off, the relay drops out and the batteries are isolated from one another.

 

I can think of two ways to make it work, if you wanted to build your own.

 

You could also make a fairly simple circuit that only connects the batteries together if the voltage on the vehicle battery is above 13V. At rest a 12V battery is rarely above 12.8V, if it goes above 13, it's an indication that we have incoming charge.

 

The other way is to use the current on the wire that goes to the charge-indicator lamp on the dash. The field excitation current for the alternator goes via this lamp (many people don't know this, if the lamp blows or the connection is open, the alternator doesn't charge). When the alternator isn't running, this excitation current is high enough to light up the lamp (and potentially trigger some kind of relay driving circuitry), but as soon as it starts charging the excitation current drops out and the lamp goes out (and your isolation relay closes).

 

Usually you don't have to worry about it. You get commercial relays designed to do this. You fit one of them and Bob's your uncle.

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Hi, 

 

 

My question is: would this not be bad for the two batteries? I don't think the AGM battery would have enough "cranking Amps" to start the 2.8 Turbo Diesel engine, which is why there's a car battery as well. 

 

Any thoughts?

The 102Ah AGM battery should have no problem starting the 4x4 but it is only used for auxiliary appliances, they are not interconnected when the engine is not running. Otherwise after a few days camping the 4x4 may have trouble starting.

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The 102Ah AGM battery should have no problem starting the 4x4 but it is only used for auxiliary appliances, they are not interconnected when the engine is not running. Otherwise after a few days camping the 4x4 may have trouble starting.

My question was more related to the fact that two different batteries are used, and they're possibly a different age. This is controversial to solar where one wouldn't use two different (age and rating) batteries. 

 

This brings me to another question, what if the same principles (with the relay) is being used in a solar setup where two different batteries are used, in the same bank?

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The issue in a solar bank, far as I understand it, is that you create two paths by which current can flow, the path of least resistance is where most of it goes. So when you have dissimilar batteries in parallel, one will take more charge and fill up faster than the other, and then the only way to make sure your second battery is also fully charged is to continue charging, thereby potentially overcharging the other battery. Likewise when you discharge, one bank discharges faster than the other, cycles deeper, and therefore ages faster.

 

So you end up with a worst-of-both-worlds setup, where your new/bigger battery ages faster because it works harder, and the old/smaller one ages faster because it is never properly charged.

 

The relay in a vehicle only ensures that the vehicle battery doesn't discharge, but it cannot guard against the overcharge issue mentioned. Lead-acids can handle a bit of overcharging, so as long as they are not too dissimilar the problem isn't too big.

 

Also -- and once again this is controversial -- depending on how you do this, placing strings in parallel might not be as big a sin as some think it is. For sure it definitely has a negative effect on the total life of a battery, but there are cases where that might be okay. The reasoning behind this is simply that if you discharge a parallel set of dissimilar batteries, then initially most of the charge comes from the stronger string, but as that string discharges the weaker string will slowly start to pick up and if you discharge do a good DoD they should eventually deliver charge in proportion to their own capacities. There's actually some research with a proper paper on this topic:

http://www.battcon.com/papersfinal2002/mcdowallpaper2002.pdf

 

So far I haven't managed to find a plausible scenario for a combiner in solar. I think what would make the most sense is if I could charge them separately, thereby avoiding overcharge, then combine them on discharge. While the batteries naturally, because of their chemistry, proportion discharge according to their capacities, they don't do it on charging, so the challenge is to distribute your charge over the two batteries in some balanced way. I don't think it's impossible, but it's probably not worth the effort.

Edited: The top paper references this one, but this is the one I was really thinking of:

http://neuralfibre.com/paul/wp-content/uploads/2007/05/can-we-now-sin.pdf

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I understand what you're saying w.r.t the under/over charging of batteries, which is why you shouldn't mix them. But, I still wonder if there is a way around this, apart from using multiple smaller charge controllers, connected to each battery individually - which would probably cost a small fortune on it's own. 

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The trick is going to be to introduce small adjustable resistances in the various paths to control the flow of current, to the amount of a few milli-ohms. All of this is easily doable as long as you don't expect to run a load of the battery at the same time :-)

 

If I had to design an inverter that could to this, what I would do is make two separate input stages that runs off separate battery banks, then take the combined current of these input stages and feed it into a single output stage. A bit like this:

 

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