How to turn a 14v PS into a car batter charger?  - Page 1
Forum section: electronics / projects

author: WacoAvionicsTech
author_rank: Newbie
timestamp: June 25, 2018, 02:52:44 am
content: I am looking for as simper as possible to circut to add to a 14v ps so it will not overcharge the car battery.I would like to build an add on circuit that has ovecharge protection and a charged indicator.Thanks

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2018, 03:00:47 am
content: I think it's not easy. Car batteries get charged to 14 V or more. A 12 V PS won't have a high enough voltage without boosting it.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 25, 2018, 03:02:42 am
content: With “only “ 12 volts you will never even fully charge a Pb-acid battery, much less over charge it.Please search on the web the Battery University, and have a good read how to properly charge a battery.

author: WacoAvionicsTech
author_rank: Newbie
timestamp: June 25, 2018, 03:03:21 am
content: I simplified the statement...my psu can go up to 17 volts....Thanks for reminding me about the higher voltage on car chargers.

author: digsys
author_rank: Unknown
timestamp: June 25, 2018, 03:11:55 am
content: Quote from: WacoAvionicsTechThen, either set it to 13.8VDC permanent for trickle charge, or manually set it to ~14.6-14.8VDC for when it is flat = boost charge.Otherwise, you'll need something fancier

author: WacoAvionicsTech
author_rank: Newbie
timestamp: June 25, 2018, 03:14:39 am
content: I would like to build an add on circuit that has ovecharge protection and a charged indicator.

author: digsys
author_rank: Unknown
timestamp: June 25, 2018, 03:54:19 am
content: Quote from: WacoAvionicsTechMy favourite SLA+ battery charge / management IC is the BQ24450 (or similar). Easy to set up and does everything. Cheap and easy, withany functions, as you need them.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 25, 2018, 01:06:29 pm
content: Exactly!That is why I have advised the OP to review the different Pb-acid charge profiles and management, before he attempts to design anything.Those profiles may be as simple as trickle charging, to complex multi-stage ones. All the profiles have tradeoffs that must be considered.

author: ArthurDent
author_rank: Super Contributor
timestamp: June 25, 2018, 05:31:13 pm
content: I'm assuming you are using an adjustable regulated supply from what you have said so far. What you want to do to protect from overcharge is more complicated to do with a non-regulated supply.A couple of problems with using most power supplies as battery chargers (which they were not designed to do) can cause damage to the supply. First, regulated supplies may not like voltage being fed into them if they lose input power for any reason. The second problem is that some supplies have overvoltage protection built in so if something happens to the supply there is a 'crowbar' circuit that consists of an SCR that shorts the output out to protect the load. If for any reason the crowbar is triggered when the supply is connected directly to a car battery the power fed into the protection circuit from the battery will blow that circuit and probably destroy other parts as well.A simple fix is to connect the PS to the battery through a diode capable of handling more than the current supplied and set the PS output higher to compensate for the diode drop. Schottky diodes have a lower forward drop and would have to dissipate less heat. Using a regulated supply with the output set to 14.1 VDC so with the .3 VDC drop from the diode the proper 13.8 VDC float charge will be fed to the battery. Depending on how high the output current is you may want to have the diode on a heat sink. If you need to buy one there are lots of power supplies on Ebay that would work for you and probably a 15 VDC one would have enough range so it could be set to 14.1 VDC output.