EvanR
HalfDork
5/7/13 1:03 a.m.
At work we use 12V, 35Ah, sealed lead acid batteries. When they load test under 20Ah, they're pretty much free for the taking 
Okay, so I want to take one camping with me, to run 12V LED lighting and a phone charger.
I need some way to recharge it using my car's charging system while driving about during the day. This would be a temporary system.
Either this is as simple as running a fused line in parallel to the car's battery, or it requires some sort of complicated, regulated charging system.
Who has an answer for me?
Well, I'm not an EE, and most of what I work with has a lot more than 12VDC, (24 to 48 VDC is what I play with), but I think a simple fuse protected charging circuit will do the job. Your cars charging system shouldn't be running higher than 14.5 v, and is more likely running around 14v even.
Keep in mind that your not going to be replacing your cars big high AH battery with these little ones and expecting them to take all the power your charging system can offer. Your just putting this comparatively small battery load in parallel to the existing battery.
If your worried, just check and see if they get hot after an hour of driving.
.
You probably need one of these.
A battery isolater will keep one battery from sucking the other dry. The only other problem would be dumping high charge amperage into a small battery.
NGTD
Dork
5/7/13 8:19 a.m.
Get a battery isolation switch for a boat. They have positions for:
-
Off
-
Charge both
3 and 4. Charge one or the other
How about you just plug it into a battery charger for a couple hours before you leave for the camping trip?
EvanR
HalfDork
5/7/13 12:30 p.m.
Sky_Render wrote:
How about you just plug it into a battery charger for a couple hours before you leave for the camping trip?
That works for day 1. What about the other 4 days?
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I'm amused. I basically got 2 responses. Either {a} do the absolutely cheapest thing, or {b} do the most expensive thing.
This is why I love GRM!
I believe that you can wire your multiple batteries parallel for charging and install a switch between the "camping load battery" to keep it form draining your "start the car again" battery
it would look something like this:
alt charge ------car bat positive -------------camp bat positive
Ground ----------car bat ground ----\sw----camp bat ground
You could then hook up your radio, lights, and power inverter directly to the camp battery pos and neg terminals (or use terminal strips, cigarette lighter receptacles, etc) and as long as the negative switch from the car battery to the camp battery was in the "OFF" position it should be electrically isolated from the car. Then when it came time to charge the battery again you could turn the car on (do that first) then turn the switch "ON" to reconnect it to the car's charging system.
I'm thinking this would work (although I've never tried it) because I used to make a killing bringing old optima batteries to life using a similar method with a battery charger. What I would do when a battery was so dead it wouldn't charge was to take a good battery and wire it parallel to the dead optima battery. This would trick the charger into thinking it had a good reference voltage. I would then hook up the charger and turn it on for a couple of hours. After that the dead Optima was usually juiced enough to provide the charger with a good reference voltage so I'd remove the good battery and continue charging.
Good times man, and good luck. I wanted to do a similar system when I was back in high school. I had a GMC van we used to call the "Port-O-Party".
Cheapest: Cut the end off a cigarette lighter plug adapter thingie. Stick + on +, - on -, run car/charge extra battery.
Not that cheap but pretty easy: Get HF inverter in the 150-ish watt range. Get small battery charger, like 1 amp or 3 amp. Plug into HF inverter, inverter into car's system. Profit.
