# Max current from fully charged battery to a dead one?

#### 2000Trooper

I’m installing a dual battery system in my vehicle. There will be the starter battery/alternator combo and a secondary auxiliary battery that will charge VIA a 30A DC-DC charger that will turn on upon detecting alternator voltage. Now, if I may say so myself, I’m over engineering this. 4 AWG cables will be used. I know it’s over doing it but in the process I will install bus bars for possible future expandibility.

The 4AWG should handle 200A draw without issue, which is more than what my 2.2kW starter would draw. I’m thinking about installing an emergency bypass switch so that in the event the starter battery fails I can start the vehicle with the aux battery.

What I’m curious about, which I don’t think will be an issue, but I’m curious nonetheless is when you have a dead battery and a fully charged one, how much current flows between the two? I’d imagine the immediate inrush current is probably very high. Short of testing this myself it’s not something I’ve been able to find a lot of information on.

What vehicle and engine?

That depends on too many factors. If the battery is very low, it not take much current at all until it starts to take some charge. How much that current peaks depends a lot on the condition of both batteries, the resistance of the cables, etc.. Not enough info.

Use Oms law. The current that will flow is = to the voltage difference between the two batteries divided by the total resistance (resistance of both batteries added together + resistance of wires and connections).

However, though I have never even seen one in person, I remember looking at adds for a system to convert a single battery vehicle to a duel battery system. I think there are various ways you can use two batteries. You can have one dedicated to the starter, and one to accessories (such as but not limited to a very powerful stereo system). You could include a switch to select which one you want to use for the starter, and/or you can rig a switch to tie them in parallel for starting in very cold conditions.

I looked in my Summit catalog and did not see anything for working with a two battery system, but I am pretty sure there are items made for these kind of systems.

In general if you are going to actually tie two batteries in parallel, it is a very good idea to use identical batteries the are also the same age. If I were going to tie two together I would probably initially tie them with something with some resistance such as a 12 Volt car light for several hours to let them equalize in Voltage.

I remember that it was a video, that there was someone with a truck with a duel battery system and they were commenting that they had to replace both at the same time. (I think he commented, that for that truck it was common for rain to drop from the back side of the hood onto the firewall and then to the top of one of the batteries and over time too much rain water getting into that battery ruined it). Also it is not good for the connections.

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What I’m curious about, which I don’t think will be an issue, but I’m curious nonetheless is when you have a dead battery and a fully charged one, how much current flows between the two?
It won't be a lot of current. A discharged battery will charge at around C/5 at 12.6 volts. For a 60 Ah battery, that's 12 amps. The voltage won't be as high as 12.6 volts either, since the voltage of the charged battery will drop under load, so it'll probably only be a few amps.

Usually secondary batteries use an isolator (& the auxiliary one is a deep cycle to power accessories). That way the alternator can recharge both of them when the engine is running. Unless you wanted 2 starting batteries (like a diesel), then they’re just tied together in parallel.

They make a battery switch for boats that will allow you to use either 1 or both at the same time, they also make an electronic switch for 2 battery systems that is used on RV's. You normally have a starter battery and a house battery. Both are charged during engine run time from the Alt. and there is a switch on the floor like the old headlight high beam switch. If the starter battery is weak you press and hold the floor switch which will connect the house battery to the engine.
As for connecting a weak battery to a fully charged one the current flow will be very low BUT you need to be careful that the low one does not have an internal short or the current will be the max the good one can provide and can cause the shorted one to explode.

it is a very good idea to use identical batteries the are also the same age
agreed, using the same batteries of the same age is a very important thing in a dual battery setup

What battery chemistries are both batteries?

A DC to DC charger is usually only used with lithium chemistries where the charge rate of a dead secondary battery is so high that it can overheat the alternator. If your secondary battery is a lead type, you typically don't need the DC to DC charger. A better option is a voltage operated isolator like this:

I saw a battery explode once when someone jump started his own truck. I was across the street in the house I lived in and was sick that day so I was just on a couch watching through a glass door. Imagine about a gallon of acid exploding up in all directions with enough that you could actually see the amount of fluid in the air. Besides the direct hit of the acid it also bounced off of the bottom of the open hood and some of that also got him from above. He ran into the house and under the shower. Fortunately for him all of those houses were single floor and small so he did not have run far. I talked to him the next day and he said he had connected the polarity correctly and it still blew up. Though I did not at the time think to ask if the last connection was far away from the battery (which it should be to avoid igniting any hydrogen fumes around the battery).

It won't be a lot of current. A discharged battery will charge at around C/5 at 12.6 volts. For a 60 Ah battery, that's 12 amps. The voltage won't be as high as 12.6 volts either, since the voltage of the charged battery will drop under load, so it'll probably only be a few amps.

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Perfect explanation, thanks!

What battery chemistries are both batteries?

A DC to DC charger is usually only used with lithium chemistries where the charge rate of a dead secondary battery is so high that it can overheat the alternator. If your secondary battery is a lead type, you typically don't need the DC to DC charger. A better option is a voltage operated isolator like this:

I thought about using one of those but it turns into a hassle since I have winches and I’d have to gauge up for heavy draws. Also the alternator only puts out 13.6V since the factory battery is a FLA and the aux battery is SLA which requires higher voltage. Hence the DC-DC charger. The DC-DC also serves as my MPPT.