SLA floating in a car - Page 1
Forum section: electronics / beginners

author: viperidae
author_rank: Frequent Contributor
timestamp: July 24, 2019, 08:03:52 am
content: I'm putting a computer in my car, and to get around the issues of battery draining if something goes wrong and surviving engine cranking I though I'd use a 7.2Ah SLA battery I have laying around from an old UPS.I figure I can run the computer from the SLA battery and connect the SLA to the switched car supply via a diode.My theory is it will be kept at half a volt lower than the flooded lead acid battery in the car. so shouldn't suffer too much from the ~14V when the alternator is going.Anyone see any flaws in my plan?I'm a bit paranoid about having something go wrong and returning to a car after work with a dead battery.In terms of power delivery, I'm expecting no more than 20W peak.

author: cs.dk
author_rank: Unknown
timestamp: July 24, 2019, 08:09:34 am
content: Why not a relay driven of the D+ signal (engine running). That way, if the little battery fails, it can't drain the main battery, and it will recieve a proper charge.

author: viperidae
author_rank: Frequent Contributor
timestamp: July 24, 2019, 08:17:28 am
content: I didn't think the SLA would like being effectively float charged at the 14+V of the alternator when it's charging the car battery, hence the diode.If I power the set up via a circuit that is only power when the ignition is on, I will never have to worry about a flat car battery

author: cs.dk
author_rank: Unknown
timestamp: July 24, 2019, 08:25:45 am
content: Quote from: viperidae on July 24, 2019, 08:17:28 amThats why i suggested a relay or other circuit running of the D+ signal, instead of KL.15 (ignition). Thats how most european campers are made. You can still install a diode in series if you want.If the second battery is flat in a camper, and you turn on the ignition, it can drain the main battery. If the relay first pulls in when the engine is running, the alternator is there to help you out.

author: viperidae
author_rank: Frequent Contributor
timestamp: July 24, 2019, 07:04:40 pm
content: I'm not aware of a signal for "engine running" in my car. It's a gen 7 JDM Honda Accord. There is canbus data that can tell me that but I don't think there is a wire for it.

author: Wigo
author_rank: Contributor
timestamp: July 24, 2019, 07:11:12 pm
content: There are dedicated CarPc-Powersupplies out there. They take 12V input, have an Atx connector and are switched of and on by your car via a signal wire.It is nearly a decade since i have installed a Pc in my car but that was what i have done.The "signal wire" is nothing more than the ignition signal running into the car stereo, or if you want it nice you could use your remote wire on the stere for it.But i have to admit i do not know how your Honda is build up in this way.

author: bdunham7
author_rank: Super Contributor
timestamp: July 24, 2019, 08:05:23 pm
content: Quote from: viperidae on July 24, 2019, 08:03:52 amYour plan is fine except that a dead or shorted SLA battery could draw more current than allowed for whatever circuit you connect to and blow a fuse. I'd suggest running it off the AUX circuit or it's equivalent--this would be the same branch that the HVAC and radio are one--and using a current limiting device which could be a PTC fuse, a circuit breaker or even a properly heat-sinked resistor. Aim for 3-5 amps max current.

author: wilfred
author_rank: Super Contributor
timestamp: July 25, 2019, 01:34:07 am
content: What is this computer going to do that say a tablet couldn't do? I can't help thinking you're solving the wrong problem. I am assuming you will need a screen of some description anyway.

author: viperidae
author_rank: Frequent Contributor
timestamp: July 25, 2019, 03:41:12 am
content: Currently the computer is a raspberry pi 3. The screen is a 7" touch screen. There will be a micro controller, a few can bus interfaces, maybe a 2.5" hard drive, an rtl-sdr dongle. Haven't decided on anything else yet but I'll be replacing the entire navigation system that is intertwined with the radio, HVAC system and half the dash lighting.

author: GerryR
author_rank: Frequent Contributor
timestamp: July 25, 2019, 02:02:43 pm
content: You could use a small power inverter and plug a wall-wart supply directly into it for your computer. The inverter can plug into a power socket or be hard-wired into you 12 V system. Just a thought.

author: Muttley Snickers
author_rank: Unknown
timestamp: July 25, 2019, 06:44:56 pm
content: Quote from: viperidae on July 24, 2019, 08:03:52 amI always keep the same capacity batteries in stock and routinely carry a charged one around in my truck just in case a pretty lady needs a quick jump. I have no problem jump starting a V6 Toyota Hilux with those batteries either so certainly another backup plan worth considering.