Reducing the Current output of a battery charger - Page 1
Forum section: electronics / repair

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: October 24, 2021, 04:35:05 pm
content: Hi guys,I have a battery charger that is a 12v / 24v (auto select) charger that puts out 8 Amps at 12v and 4 amps at 24vBut that is too high for me.I'd like at most 2 Amps @ either 12v or 24v. I think ideally 1Amp output would be best overall.I'm thinking resistors are the way to reduce current without reducing voltage but Not sure how to do it.I can provide a full schematic if that would help?Here is a picture if the internals of the charger and a list of the major components.So how do I reduce the current on the output of both the 12v and 24v down to 1A ?PS....Not necessarily looking for an "exact" solution, but just in general and how roughly.Thanks

author: tunk
author_rank: Super Contributor
timestamp: October 24, 2021, 04:49:29 pm
content: Wouldn't really have a clue, but above the the twocapacitors to the left there's something that couldlook like a current shunt. You could possibly try toreplace it with:- either four times the length and same diameter- or same length and half the diameterOr it may blow up your charger.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: October 24, 2021, 06:26:12 pm
content: Quote from: tunk on October 24, 2021, 04:49:29 pmThat's the best suggestion so far. I'll look into it.Thanks!

author: abdulbadii
author_rank: Frequent Contributor
timestamp: October 25, 2021, 07:50:43 pm
content: If it's really, frankly I don't knowbut first we must ensure its claim is true by good, reliable, accurate MMRead the charger current when it's charging by putting the batt. on end to the charger the other popped up to open to be used for MM's probe to meter as in series with the other charger end.if it requires 2 batt. to be viable to charging, put another batt in other slot or just short this slotRead this current, really as the rate stated above ?

author: BrokenYugo
author_rank: Super Contributor
timestamp: October 26, 2021, 01:16:14 am
content: Does "motorcycle" mode not do something like that already?

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: October 28, 2021, 11:31:59 pm
content: Quote from: tunk on October 24, 2021, 04:49:29 pmTurns out the wire jumper/shunt measures less than .1 ohm so I don't think that will have anything to do with reducing current.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: October 28, 2021, 11:32:34 pm
content: Quote from: BrokenYugo on October 26, 2021, 01:16:14 amGood question.I hadn't tried that mode but if so you may be onto something.But I can't right now because I have it torn down in the lab recreating the Schematic and PCB in KiCAD so I can make changes to suit my needs.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: October 29, 2021, 06:18:44 pm
content: I'm thinking the way to do this is by adjusting the timing of the PWM cycle.That would be done primarily by changing resistors (I believe).This should be easier to figure out once I have the schematic completed. There's gonna be some math involved.

author: BrokenYugo
author_rank: Super Contributor
timestamp: October 29, 2021, 07:02:27 pm
content: I point out motorcycle mode because I assume the goal here is to charge small SLAs, which many motorcycle batteries are, I've seen them as small as 3-4Ah.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: October 30, 2021, 12:02:26 am
content: Quote from: BrokenYugo on October 29, 2021, 07:02:27 pmWhen I put it all back together, I'll try that mode.I makes sense. Why have a "motorcycle mode" if it's the same output parameters as the full size car / truck battery?

author: bdunham7
author_rank: Super Contributor
timestamp: October 30, 2021, 12:27:34 am
content: Quote from: ThermallyFrigid on October 28, 2021, 11:31:59 pmI think the point is that the shunt is used to measure the current, then that measurement is used to control the current, as well as display it. So increasing the shunt resistance 2X would cause the charger to put out half the current, but probably not display that correctly.If you have a motorcycle mode, that should be 2A maximum.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: October 30, 2021, 11:03:44 pm
content: Quote from: bdunham7 on October 30, 2021, 12:27:34 amI can try that just to see.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: November 02, 2021, 10:34:00 pm
content: The only viable way is to acquire schematic by contact to the maker/seller or doing RE, get all componets specs.Then if not experienced enough try to install and run LTspice (OS: all. On Linux need Wine), and simulate it around either the Voltage regulator or zener diode functioning to regulate output volltage as long as within certain R load range it is operating. Get this R range specs of the either regulator of output, by we know.. digging info, googling.Find R min of this R range, this would be the sum of R internal battery, original load, and another R load in parallel to the R batt. R.min <= R.batt || an R. Get this R =1/ R >= 1 / R min - 1 /R battApply the R parallel to battery, so that it will shunt and might reduce the current in original load ie. for battery, means get so many more the same type battery, calculate R.batt || of all them and charge them all parallel at once. Each has charge with decrease current going throuh iIn case haven't known how to get R internal batt,Try meter batt DC volt and DC current it generates by quite good, accurate MM, and simply divide both in that order i.e. batt DC Volt / I

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 03, 2021, 10:13:15 am
content: Quote from: abdulbadii on November 02, 2021, 10:34:00 pmWhat?? You only need to find the shunt resistor. There's always a shunt resistor or current metering method.Then, find the IC that measures this., the datasheet will show the drop voltage->current conversion formula, all you need is to calculate the new shunt resistor and replace it.ThermallyFrigid, do you really think we can help you with only these pictures?It's like if my engine breaks down and ask how to repair it showing a generic car pictureMake proper pictures showing the component markings, the bottom traces...That modification is usually veryeasy, but we need to see how it's made.I did the same few years ago to charge a small 7A lead acid battery with 5A charger.Added a switch to lower the current to 500mA, worked nicely.