Tank battery & ventilation; run lines carrying high voltage

**PredatorCoder** · 02-13-2006, 12:42 AM

OK so I am thinking about using a tank battery on my 97 Honda Civic to avoid low voltage problems during crank:

1. Battery would probably go in trunk.
- Don't batteries need ventilation? I have heard of people putting tank batteries in trunks, but not really any details on ventilation. I really don't want my trunk exploding due to a buildup of hydrogen gas.

2. This battery will not only be running my car PC (either thru inverter or DC-DC power supply, not sure which yet), but also other accessories in the car. Additionally I will be running 110V AC lines from the inverter to regular wall outlets throughout the vehicle, so passengers can plug in any reasonable AC accessory.

So there is the potential of a high load (say 50 amps). What do I need to consider when finding a battery to handle a 50 amp load and run it for up to a minute? I want all accessories to be able to remain powered for up to a minute. It seems to me that amp-hours is not the only thing to look at, because the load is high (50 amps), vs. the < 10 amp load typical of just a VIA mini-ITX computer.

3. Lines carrying 110V from the inverter are obviously quite dangerous. How would one go about *safely* running these lines to AC plugs throughout the cabin? Obviously you do not want these to short against the frame of the car under any circumstance!

4. After looking around, it seems to me that the simplist circuit would be wiring the main car battery and the tank battery in parallel, with a diode between them. This would then get a nice 12V source to the computer while cranking, right? (What's the point of the resistor and second diode I've been seeing? I don't quite understand... is it really necessary?)

**roadhog** · 02-13-2006, 11:13 AM

1) Depends, most modern car batteries are sealed. Look for "Maintainence Free"
2) Car batteries are designed for loads like this.
3) Fit the cable in appropiate trunking or ducting. Ensure the cable is heat rated for the load whilst in trunking/ducting.
4) The cable between the batteries should be capable of carrying 100a @12v minimum, directly wired, or 50a @12v if using a split charging relay. I recommend the latter. I can't comment on resistor/2nd diode, haven't seen it.

**PredatorCoder** · 02-13-2006, 02:33 PM

1. I was under the impression that even an SLA battery will need some amount of venting - they do release gasses under certain circumstances, right?

2. Right, but obviously there's a limit. I don't want to spent $100+ on some Optima SLA battery - I'm not trying to keep stuff running for hours. I'm just trying to: (a) maintain a steady 12V even while the car is cranking, (b) provide 1 - 2 minutes of power after the car is turned off. Supposing two minutes, that is 0.0333 hrs. Multiply that by a 70 amp load and you get 2.33 Ah. So is that really all I need to do, just go buy a 3 Ah battery? The back of my head says that a 3 Ah battery probably is not designed for a 70 amp load, but am I wrong?

4. Using a single diode between batteries, would result in the tank battery getting a constant 13.9 voltage or so on my vehicle when the car is started. Any reason this would be a problem or is this an acceptable way of charging any 12V SLA battery?

**roadhog** · 02-13-2006, 03:09 PM

1. It's quite possible they do but your boot isn't airtight. It has 2 vents to allow cabin air to exhaust.

2. Car batteries are designed to operate heavy loads for relatively short periods. An "average" cold start may require loads of 200a @12v for cranking - hence why the main battery may volt drop. I haven't seen any 3Ah batteries with terminals which would safely cope with 70a @12v... so I guess they're not generally designed for it.

4. Depending on the state of charge between the two batteries there may be considerable current flow between them. You would need to check the current (and reverse or bias current) rating of the diode. The conventional method of doing this before CarPCs was to use a split charge relay and I see no reason to change.