Current through a relay between two batteries

**RFMan** · 06-04-2005, 02:01 AM

Hi,

I'd like to get some feedback from people that are using a secondary battery connected to the main through a relay.

My main concern is the amount of current flowing through that relay.

I looked up the internal resistance of an Optima Yellow Top battery, and it's rated at 0.0028 ohms. Lets say two of these are in a car, the main battery which would be at 13.8V with the car running, and the secondary battery, which had been discharged down to 10.5V because I ran my amps for a good while.

When the relay is tripped, I would have a voltage difference of 3.3V flowing through a total resistive path of 0.0056 ohms (the resistance of both batteries). That's 589 amps of current. There's no relay that can handle that kind of current, not to mention that instantaneously switching that kind of current is not what I consider safe in a car.

So do you guys have a resistor in series between the batteries? I've seen some 25W (and even 225W) resistors that could limit the current to a few 10s of amps (light bulbs are also great at doing this for temporary setups such as charging a very large capacitor), but that would prolong the recharging time of the secondary battery greatly...

Any feedback is apreciated.

Luc

**Jahntassa** · 06-04-2005, 07:20 AM

I think this is why most setups (in car audio) with dual batteries use solenoids, or isolators specifically for the task. They'll also have something like 2 AWG or 1/0 AWG wire between the two, something a little 30/40 amp Bosch relay wouldn't be able to take.

Try looking into the solenoid concept, I think that's the safer (and less mental thought) way to go.

**NotchbackVW** · 06-04-2005, 11:42 AM

I dont see the problem. Lots of modern security systems use automotive relays to cut out the feed to the starter solenoid.

It depends on your application, if you are using it to connect and disconnect charging / cranking i dont see any issues with a 40amp relay.

I have done no maths on this, just my logical thinking, and compared to standard car applications. My altenator is 70amp (max!) and i have two batteries so will each be getting 35 amp max (below rating of relay?)

Also i have used a relay to seperate my two batteries with igniton on off for a year now with no problems. Although it does get warm!

PS. I take no responsibility if you set fire to your car!

**NotchbackVW** · 06-04-2005, 11:45 AM

if your worried, i know it seems bodgy but why not use two relays in parralel?

Half the amps through them.

If your not cranking through the relay, only charging through it then make sure the total amps of relays exeed the max output of your altenator and you know your safe.

**meddler** · 06-04-2005, 11:58 AM

Originally Posted by NotchbackVW

I dont see the problem. Lots of modern security systems use automotive relays to cut out the feed to the starter solenoid.

That's not quite correct. In most cars they interupt the start switch to the solenoid on the starter. The solenoid connects the starter to the battery, not the relay that is switched by the key.

You use a starter solenoid to connect one battery to the other. They should only be approx $10-$20. I have one that is rated to 200A but my alternator is only 55A (80A after I upgrade it). The max current to the second battery can end up being quite high but it is usually not a problem because it is for such a short period of time. The current draw will taper off as the secondary battery voltage matches the other battery. The same thing happens when you charge a battery in a stock steup. The alternator puts out high levels of curretn initially and then it gradually decreases.

**Bugbyte** · 06-04-2005, 01:37 PM

Guys, check out "Tank Circuits" in the FAQ Emporium.

**NotchbackVW** · 06-04-2005, 02:25 PM

Originally Posted by meddler

That's not quite correct. In most cars they interupt the start switch to the solenoid on the starter. The solenoid connects the starter to the battery, not the relay that is switched by the key.

Maybe, but not always. Bear in mind i only have experience of cars over 25 years old!

So there is not a relay between the ignition switch and the starter solenoid!

Having said that the security systems I have installed arnt 25 years old and several have clearly stated to splice the relay (standard 40amp) into the main feed to the solenoid, preferably somewhere hidden, as this is the best for security.

Using a solenoid to join your batteries is a good safe and reliable option. But IMO not always necassary.

**meddler** · 06-04-2005, 09:57 PM

not all older cars have a relay on the solenoid switching line, that is true. But I wouldn't want to have a 30A relay on the main power line from the battery to the starter. Some starter motors can draw in excess of 200A while starting the car. IMO trying to draw 200A (even 80A if you have a smaller engine) through a 30A relay isn't a good idea.

**RFMan** · 06-04-2005, 11:30 PM

Originally Posted by NotchbackVW

I have done no maths on this, just my logical thinking, and compared to standard car applications. My altenator is 70amp (max!) and i have two batteries so will each be getting 35 amp max (below rating of relay?)

Actually, my concern has little to do with charging from the alternator but more with the equalisation of charge between the 2 batteries. This has nothing to do with the car environment really. Whenever 2 batteries at different voltages are connected together in parallel, the stronger battery will charge the weaker until their voltages are equal and a very substantial current can form in the wire connecting them, albeit for a very brief moment.

The starter solenoid is a very good idea though, and it may end up being the solution of choice here.

**NotchbackVW** · 06-05-2005, 07:37 AM

Originally Posted by meddler

not all older cars have a relay on the solenoid switching line, that is true. But I wouldn't want to have a 30A relay on the main power line from the battery to the starter. Some starter motors can draw in excess of 200A while starting the car. IMO trying to draw 200A (even 80A if you have a smaller engine) through a 30A relay isn't a good idea.

Obviously i wasnt suggesting a relay in the main wire from the starter motor to the battery, I aint that daft!!!! But from ignition switch (or relay) to the starter solenoid. Thats what i said didnt i?

RFman go with the solenoid solution if your worried, best solution if it means it doesnt play on your mind.