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Whats the best way to maintain a 12v SLA battery in an automobile?

Would a direct 12v connection to the vehicles battery system be sufficient, or is there a circuit that should be placed in between for maximum health retention?

  • 1
    Can you explain your goal? If you direct connect, then anything that causes the main battery to die will also cause your stored battery to die. If you want to have the battery as a backup starter battery, you have to find a way to disconnect it when the car is off. If the car will be parked in the sun occasionally, you could use a small solar panel to keep the battery full. – mkeith May 1 '15 at 0:26
  • The goal is to have a backup battery for starting a vehicle in an emergency. If I put a diode on the line I suppose it would protect against power draining. Any other recommendations for the charging circuitry? – Dobler May 1 '15 at 1:23
  • 2
    Can you go through the scenario in full detail? We don't know how much you know, and some people have some very low level misconceptions. You could basically buy a replacement starter battery for your car, put the battery in the trunk (boot) and keep it topped off by connecting it with a diode. But then when you need to USE the spare battery, you basically need to remove the old one and install the spare. There are also some serious wiring and fusing issues you need to consider. – mkeith May 1 '15 at 16:09
  • Please edit your question to make it more clear what you are asking – Move More Comments Link To Top May 4 '15 at 15:19
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First of all, SLA is not the optimal kind of battery for starting an internal combustion engine. SLA are optimized for deep discharge, but their output current has been sacrificed. You need a starter battery (like the first one that's already in the vehicle). Starter batteries are designed for high current output (the cold cranking amps).

Dual battery systems are fairly common on: boats, RVs/caravans, emergency vehicles 1. There is usually a battery isolator, which prevents the secondary battery from draining the starter battery and vise-versa. There are 2 classes of battery isolators:

  • Diode-based
  • Relays-based (solenoids)

Either kind of battery isolator would work for your case. Relay-based is more convenient, I think.

In your case where the second battery is used as a backup starter battery, you can disconnect the backup battery simply with a manual switch. Normally, the backup battery is disconnected2. Connect it to the alternator for a couple of hours once every so often to top it off.

The charging current doesn't require a hefty wire, but the starting current does. So, either provide the hefty wire, or carry the backup battery closer to the starter and use jumper wires, when the main battery is dead and it's time to put the backup to use.

1 In these cases, however, the second battery is for powering appliances. In those cases, the battery is indeed an SLA.

2 If it's not disconnected, it's not a backup. tm

| improve this answer | |
  • +1. While we're at it, there are a number of neat little jump starters with air compressors, power inverters and phone chargers built in, whose prices compare well to purchasing and carrying a second automotive battery. Running cables through the firewall isn't much fun. – Sean Boddy May 1 '15 at 4:16
  • 1
    I disagree about the charging wire. The alternator current may not be too high, but the other battery is also attached. When you connect two car batteries in parallel, very large currents can flow if the two batteries are at slightly different voltage levels. Proper wiring and fusing is important. – mkeith May 1 '15 at 16:12

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