Second battery: Any caveats?

Question

I have a 2004 Hyundai Sonata. I'm a volunteer with a fire department, so I have some additional equipment in my car - a two way radio, a scanner, an emergency light, etc. Note: Most of these are off when the car is off (although wired 'hot'), they may have a small drain when they're off.

Lately I've had issues with the car starting - overnight only, really. If I'd drive somewhere at 8 AM then leave at 5 PM, it's fine. But then the morning, it's a problem. One of my radios has a volt meter that displays voltage when powered up, and one morning, I checked, and it showed 10.2 volts - the car wouldn't start. So had the system checked out - battery is fine, starter fine, alternator fine. Only suggesion is the radio scanner etc are drawing enough current to drain the battery overnight.

So had a second battery installed. Everything aftermarket now runs off that battery. That battery will only charge when the car is running. If that batttery dies, the primary battery isn't affected.

So my question - is there anything I need to worry about now? Is the alternator going to be doing double duty, charging the second battery or anything? I forgot to ask at the shop, and won't be back there for a few days. :)

Thanks!

Paul in Houston TX · Accepted Answer

Odds are B2 won't draw enough from the alternator to worry about. What is the amp draw when everything is "off"? At one amp draw it would take a long time to run down a battery. How old is B1?

Evan Platt · Answer

Wish I knew. I bought two clamp multimeters, neither works as intended. I'll have to try a regular multimeter sometime.

Well, how long would it take to run down the battery to the point of being able to 'run' but not enough to start?

3-4 months. But it's been jumped about 5-6 times, so not sure if that affects the life of the battery...

AD · Answer

yep. deep discharge takes a life out of it

Scott Dorsey · Answer

Get the meter and measure it. Break the battery connection, put an ammeter in there, leave it for a while and see what it's reading. Then you know if that really is the problem.

If they did this properly, with a diode block that is well-installed, everything will be fine. I have seen some pretty awful installs done by car stereo bozos, though.

It won't need to do any more charging than before because you won't be using any more current than before. It's still having to produce a lot of current, though, because in the end you're using a lot.

--scott

Steve W. · Answer

I run the same style set-up. You say MOST of them are shut off? Which  ones are you leaving ON. And what two way are you running? Some of those draw quite a bit of  power if left on.

Bill · Answer

The second battery should be a deep cycle battery.

For now I would just use the system as provided. But if there are problems, note for some vehicles you can get a larger amperage alternator than factory supplied.

To learn more about second battery systems, search google.com for the words...

boat second battery or marine second battery or RV second battery

A LOT has been written about this subject!

gregz · Answer

A lot of if and buts. First thing I would do is measure off currents. Even

30 ma is too much. In what condition was the battery when problem occurred. It's a safety using two separate batteries, but using two identical batteries in parallel will have the longest useful life. No battery should be run to discharge state.

Greg

Evan Platt · Answer

Is it worth seeing if it will be replaced under warranty - assuming of course it would obviously have to fail a test I'm sure...

jim beam · Answer

get a "desulfating" battery charger. they work in part by pulsing short bursts of h.f. through the battery which breaks up any [dendritic] crystal structures that may short circuit between plates. they'll not return a battery to 100%, or fix one that's too far gone, but they can rejuvenate many batteries substantially.

Steve W. · Answer

These work good. I cheat on my vehicle for winter use. I added an on board charger and a  block heater. Makes it VERY nice to go out on a call at 03:00 and have  it crank right up and instant heat!

jim beam · Answer

doesn't sound like "cheating" to me - it sounds like damned good sense.

i did a science project on this type of charger back when i was at high school - they definitely work. my truck's battery is a cheapo walmart now in its 10th year. when i first got it, it was real low and starting was "cross your fingers". i put the battery on desulfate for a couple of months when i was monkeying with that dodgy head gasket, and now, it starts the truck like a champ, even after a month standing idle.

Second battery: Any caveats?

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