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converting an auto battery charger to manual

stonefly

Junior Member
Jun 10, 2021
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0
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Can I convert an automatic battery charger to manual?

The old manual chargers work much better for my purposes. I don't want a charger putting itself into "float" mode or otherwise reducing amps to the battery. I want to be the one who decides when the battery has had enough. This is impossible with today's modern automatic chargers. There must be a way to bypass or shut off the automatic circuits. Does anybody here know how to do that?

As far as I know, it is now impossible to buy a truly manual battery charger. I have one, an old Schumacher, about 20 years old. I love it. I want another one, but I realize I must make it myself.
 

stonefly

Junior Member
Jun 10, 2021
4
0
6
Just buy a transformer of sufficient size and wire it up.
Thanks, but that doesn't give me much to go on.

I already have several automatic chargers, and they already have transformers, and rectifiers. In fact, they have everything I need. Unfortunately, they also have some things I don't want. I'm hoping to find out that there is a simple way to open those things up and cut a wire or two in order to simplify the automatic function out of existence.
 

sdifox

No Lifer
Sep 30, 2005
84,224
9,046
126
Thanks, but that doesn't give me much to go on.

I already have several automatic chargers, and they already have transformers, and rectifiers. In fact, they have everything I need. Unfortunately, they also have some things I don't want. I'm hoping to find out that there is a simple way to open those things up and cut a wire or two in order to simplify the automatic function out of existence.
Just bypass the electronics, wire the output from the transformer directly to the charging leads.
 

NutBucket

Lifer
Aug 30, 2000
26,430
268
126
My guess would be they have a switching power supply so I doubt it would be as simple as bypassing the electronics. It probably reads charge current and adjusts voltage output accordingly.

Sent from my Pixel 3 using Tapatalk
 

NutBucket

Lifer
Aug 30, 2000
26,430
268
126
The only time I believe a manual charger is advantageous is when you're trying to "bump" a battery with voltage so low an automatic charger will deem it defective.
 
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mindless1

Diamond Member
Aug 11, 2001
6,059
677
126
We would need more to go on, for you to open the patient and dissect it. I'm pretty good at reverse engineering circuits and modding them, but i have to know what the circuit is to do that.

If it switches to trickle mode it probably is a switchmode PSU. If it's just an old school transformer you do not just hook the charging leads up to its output, you hook them up after the diode(s) so it's DC instead of AC.
 
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mindless1

Diamond Member
Aug 11, 2001
6,059
677
126
Thanks, but that doesn't give me much to go on.

I already have several automatic chargers, and they already have transformers, and rectifiers. In fact, they have everything I need.
Do you have one with a large, heavy transformer, linear unregulated instead of regulated switchmode? If so you can use that, or if not, pick your target current and buy a 12V transformer.

Wire the transformer to a bridge rectifier of suitable current rating, or 4 discrete schottky diodes (as the bridge rectifier) would be less lossy and not need as large a heatsink (heatsink them), then add about 470uF capacitance for each amp current (arguable, for a lead acid battery charger you don't really need that much). A fuse would be nice to have too.

It is a very basic circuit, but without full charge current termination, if you leave the battery hooked up past full charge it will start to boil away the electrolyte (not really boil, rather electrolysis, convert the water to oxygen and hydrogen).

Example transformer:


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As far as I know, it is now impossible to buy a truly manual battery charger. I have one, an old Schumacher, about 20 years old. I love it. I want another one, but I realize I must make it myself.
Used charger from Craigslist, garage sales, ebay, etc.
 
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Paperdoc

Golden Member
Aug 17, 2006
1,981
141
106
Do NOT just hook the transformer output to your battery!! A transformer only works on AC, and it is there just to convert the household 120 VAC into a battery's 12 VAC. BUT it does not supply exactly 12 VAC either. AFTER the battery there is a rectifier system - often a bridge of 4 diodes in a heatsink - to convert to DC with significant ripple, then at least one capacitor to damp out some of that ripple. Some MAY have a small inductor as part of the ripple filter system, but car batteries are not really fussy about that. All of those BASIC items cause a small loss of voltage. Then realize that the car battery really is NOT 12 Volt - it usually reads about 13.2 V with no load on it. So overall the entire unit is designed for all those factors. THEN on the modern ones there come the Voltage Regulator systems that adjust the actual voltage applied to the battery to charge it rapidly when it needs that, and to keep only a small trickle charge current going when it is fully charged. It is those last things you want to avoid. But you really would need to understand the entire circuit in a modern charger to know how to change its design to your wants. Just bypassing selected components is likely to get you trouble.
 

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