Car battery power supply - Page 1
Forum section: electronics / beginners

author: phil4107
author_rank: Newbie
timestamp: December 20, 2012, 11:56:01 pm
content: Hi Guys,I've got a spare car battery and instead of recycling it I thought I'd use it to create a portable power supply. The idea is to put it in a box with an inverter, cigarette lighter socket, USB port for charging USB devices and 12v terminals connected directly to the battery for charging.Before I proceed I'd appreciate a bit of advice on a couple of things:I guess using a 0-30v 3A bench power supply is ok to charge the battery through the terminals though I'm unsure of the charging voltage. I'm guessing just over 12v, but doesn't a car's alternator charge at about 14v? Would I also need to limit the current?The USB output would be a simple circuit with a 7805 voltage regulator. Would there be any problems charging devices that draw high current (anything over the standard 500ma) or would I need a more sophisticated circuit?The cigarette lighter socket would be wired using an in-line 10A fuse.Any ideas or suggestions would be appreciatedCheers,Phil

author: Stephenniall
author_rank: Contributor
timestamp: December 21, 2012, 01:45:40 am
content: Hi Phil,SLA - Sealed Lead Acid Batteries from cars charge at 13.8 Volts.I'd imagine you would have to use a constant current source to charge it, Although i'm not too sure on that area. I'd guess it would depend on the Amp hours of your battery.Stephen

author: Psi
author_rank: Super Contributor
timestamp: December 21, 2012, 03:16:43 am
content: Yep, 13.8V if you want to 'float charge' it (keeping it on the charger indefinitely)For a 'cycle charge' (charge up and then disconnect) you can go a bit higher (14.4-15V) and get a faster charge time.Is this an actual "car battery" from a car? I ask because most car batteries are not sealed. This means they produce hydrogen gas while charging.It can be quite dangerous, especially in a space where the hydrogen can build up.If the mixture of air and hydrogen gets to a combustible level all it takes is a spark and the explosion will be quite serious.The spark can happen when you disconnect a terminal of flip a switch and that's the time when your standing right next to it.The risks can be reduced to acceptable levels by adding a fan. That's what they do for DIY electric cars which use lead acid batteries. You're required to have a extractor fan on the battery box to get the car registered (at least in NZ).Here's a vid of a car battery hydrogen explosion

author: phil4107
author_rank: Newbie
timestamp: December 21, 2012, 05:54:12 am
content: Quote from: Stephenniall on December 21, 2012, 01:45:40 amThe battery is rated at 43ah. I've been looking at the specs of car battery chargers and they seem to charge at a rate above the maximum of my supply so I guess this would be ok without current limiting?Quote from: Psi on December 21, 2012, 03:16:43 amYes, it's a car battery from a car (it's sealed as far as I can tell, though I'm no expert). I am aware of some risks regarding the acid (boiling, leaking etc due to excessive current draw), but the Hydrogen thing is slightly concerning. I think I'll take your advice and add a fan to be safe.I'd only use it occasionally for low power stuff and would cycle charge it.I don't think I'll bother building the USB circuit, I've found a suitable adapter on ebay

author: Psi
author_rank: Super Contributor
timestamp: December 21, 2012, 06:42:34 am
content: If it's sealed it should say on the side SLA (sealed lead acid)

author: IanB
author_rank: Super Contributor
timestamp: December 21, 2012, 08:51:31 am
content: Lead acid batteries use a CC/CV charging protocol. You can set the voltage limit at about 14 V for a 12 V battery and set the current limit to 0.1C. Since your supply only goes up to 3 A you can let that be the limit and the battery will just take longer to charge. Disconnect the charger once the battery is charged (when the charging current drops to about 0.1 A or after 24 h, whichever comes first).

author: phil4107
author_rank: Newbie
timestamp: December 21, 2012, 04:22:26 pm
content: Quote from: Psi on December 21, 2012, 06:42:34 amI didn't have the battery at hand earlier, but looking at it now there is no mention of SLA so I guess it isn't.Thanks for the charging info Ian.

author: SeanB
author_rank: Super Contributor
timestamp: December 21, 2012, 04:27:04 pm
content: Car batteries always have a vent of sorts. It can be a connection to a vent tube that leads to the lower end of the car so any vapour can be dumped on the ground and not in the engine to a labyrinth passage set in the top venting out od a series of slots or holes in the side casing. If it has a vent tube it is easy to lead it outside and get the vapour outside where it is safer. Depends of course on where you site the battery and if you have an outside wall that either has an existing vent you can use or if you are willing to drill a hole for the 6mm tube.

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 21, 2012, 10:21:36 pm
content: I wouldn't call it portable, more a kind of luggableCar batteries are just for starting, they are not made for repetitive deep(er) discharge/charge cycles. I think there are better chemistries out there that are more suitable for your purpose.

author: phil4107
author_rank: Newbie
timestamp: December 22, 2012, 12:12:22 am
content: Yeah, I'd have to agree with you on the portability aspect jeroenI just thought it may be a handy thing to have around, but given the problems everyone has noted it may not be that worthwhile.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 22, 2012, 02:44:38 am
content: For usb charging, id most certainly go for a little switcher, with a 7805 you burn a lot of energy. Just a little kis-3r33s or something.

author: ptricks
author_rank: Frequent Contributor
timestamp: December 23, 2012, 04:40:14 pm
content: Car batteries do produce hydrogen if over charged, the warning is there more to cover the liability than it is because of the actual danger. Hydrogen comes from the water in the electrolyte used in the cells, if it produced dangerous amounts of hydrogen every time it was charged it wouldn't last 2 weeks in a car, all the water would be gone.You can use car batteries to power loads, small current loads, like 10A ,are not going to harm a car battery, they are rated for much higher constant current rates, usually about 25% of the cranking rating. Charging voltages are about 13.8VDC when connected to a car, constant voltage, charging circuits in cars don't usually monitor charging current. Do not go over 14VDC charging voltage as that is when the hydrogen production can start.I have used car batteries for UPS for a long time and with the difference in cost it really is a way to save money. Oh, and for those worried about hydrogen gassing , those SLA batteries will release hydrogen too when overcharged, they all have safety vents installed, when the pressure inside is too great they vent the gas, Just like car batteries . SLA are sealed not because of gas concerns but because they can be installed in different orientations and will not leak out the electrolyte if turned on the side while a car battery may leak. I reclaim a lot of the SLA batteries that are perfectly fine once you top off the electrolyte. Over time the electrolyte evaporates out the safety vents, this is well known by the industry and the reason they say 2-4 year life span.A good part for a 5V regulator is the LM2576 , it is a switcher that only requires a diode, inductor, and capacitors and can supply 3A continuous.

author: SeanB
author_rank: Super Contributor
timestamp: December 23, 2012, 04:53:00 pm
content: Most hams use a car battery to power a linear amp, as the battery can supply the load while it will be charged during off load periods by a small low cost transformer. Thus you can have a 100A load with a 10A charger safely, provided you keep charge voltage in the correct range.

author: phil4107
author_rank: Newbie
timestamp: December 23, 2012, 06:57:20 pm
content: Funny you should mention the UPS thing, I came across it a couple of times whilst looking on the net.Thanks for the info guys.

author: UPI
author_rank: Regular Contributor
timestamp: December 23, 2012, 09:46:17 pm
content: Quote from: Psi on December 21, 2012, 03:16:43 amI have personally experienced a car battery violently ejecting the top cap and acid into my face during a jump start (I was 12 at the time). Luckily, there was a toilet nearby to wash my face and hands. I survived with only minor cuts and reddened skin.Lesson learned: Always connect the ground to the block, away from the battery, as the last step!