HELP!!!!
NWCCTV
Posts: 3,629
I have a question that is not related to a Parallax item, but can not locate the Spin Forum any longer. Is this forum no longer available?
Here's the question. I have an 18V Wall Wart charger but need to charge a 12V battery. Any suggestions on how to best drop the voltage from the wall wart so I do not fry my battery?
Here's the question. I have an 18V Wall Wart charger but need to charge a 12V battery. Any suggestions on how to best drop the voltage from the wall wart so I do not fry my battery?
Comments
Generally, to drop voltage for charging purposes, you'd use a fixed voltage regulator like the LM7812 or an adjustable regulator like the LM317. A lot depends on the type of battery you have. There are all sorts of Application Notes on the LM78xx series of regulators that show how to use them including for battery charging for either a lead-acid battery or a NiCd or NiMH battery.
Try the datasheet here.
See the bottom of page 5 here for a battery charger using the LM317.
In general with Lead Acid batteries, you have three options for charge control - [1] simple voltage limit; [2] simple current limit; [3] a combination of voltage and current limit.
Gel Cells are NOT very happy about fast charge - then tend to swell if charged too quickly and you might even burst the battery. Current limiting is determine by a fraction of the Amp-hour rating of the battery and it would be wise to not charge at greater than one tenth the amp-hour rating of a gel cell. I have ruined a few, but haven't had one actually burst.
One way to limit is to use a 13.8 volt/1amp wall wart. Your charge would never exceed 13.8 volts or 1 amp. For a 10 Amp-hour battery or larger that would be fine for a quick charge.
But if you want a slow or trickle charge other considerations come into play.
Firstly, trickle charges are usually at 13.4 VDC, not the 13.8. And secondly, the amps are dropped down, maybe to one-thirtieth of the amp-hour rating. The combination means that the charger can be left on the battery long-term (possibly indefinitely), but personally I would feel more comfortable with having a thermistor circuit added to shut down the charger if the battery gets too hot for an unknown reason.
A second LM317 can also be configured to limit current. So by using two of them you have a good scheme for smaller lead acid/ gel cells. The LM317 is usually rated for 1.5amps output and if you are using larger batteries, that may be all the current limiting you need. But I still hate the fact that nearly all the commercial chargers don't include a thermal shut down feature and all should be watched (or at least the batteries should be charged in a place that a battery burst won't cause damage - outside or on a concrete garage floor).
The guy who started Cadex has a pretty helpful overview here:
http://www.batteryuniversity.com/partone-13.htm
Whether you use a bench supply, an LM317, or whatever, I'd use it/them in the way he outlines.
I don't remember a Spin Forum.
The Sandbox is the place to ask questions that are "related to the use of Parallax products but not specifically addressed by an individual forum". Although you can also start up your own fan club and boost your post count there if you want...
Rich H
Generally, you will get a better response to your questions if you include a descriptive title with your post. "HELP!!!!" is so not descriptive, and I typically ignore threads without something more detailed than that.
-Phil
I too use gel cells for bench supply and since they are sealed, they really dislike quick charging. Probably 13.4 volt constant charge for a "12volt gel cell" is optimal.
I happen to use 8 volt 9AH gel cells, so a regulated 9 volt wall wart does a pretty good 'constant voltage charge' (should be 8.93 volts ideally, but with a 500ma wall wart one is likely to have few problems).
Just be very careful with automotive type chargers - they can really cook. Lead Acid batteries are really divided into two worlds - those that are unsealed (automotive, motorcycle, marine) and the sealed gel cell types. You can quick charge and zap the unsealed ones, they take a lot of abused. But the gel cells were never intended to do so. Mainly they were intended to be 'standby power' on a permanent trickle charge.
If you are deep cycling your lead acid batteries, you should consider a motorcycle or automotive battery. Of course you have the mess of open vents and corrosion - but you won't fry the battery on the charger.