# How to build car battery charger ?

#### Rufus Chucklebutty

Joined Mar 23, 2019
40
I only know basic electronics but if like to build a basic 20 amp transformer lead acid battery charger from reclaimed parts, they are not cheap to buy but consist of not a lot inside, a transformer, a rectifier and a tiny PCB, I can source all the parts but can I buy the PCB off the shelf?

Can I rewind a commonly found transformer like in a microwave oven to 14v?

Can I use multiple transformers to get a 20 amp output ?

#### geekoftheweek

Joined Oct 6, 2013
819
The PCB part isn't really anything you need. All you really need is a 14V supply and maybe a thermal cut out if you expect your transformer to get hot. A current overload would be a good safety feature also.

The battery itself will only take the current it needs to charge. I use a 16V 100A charger at work regularly and with four truck batteries in parallel it usually tops out around 60A. When fully drained the charger starts at under 20A then as the batteries charge the current goes up. Usually when it hits 60A for a few minutes the batteries are charged enough to start the engine.

As far as transformers go someone else will have to answer...

#### Tonyr1084

Joined Sep 24, 2015
6,881
Something to keep in mind: A transformer putting out 14VAC, if even only a single diode rectifier can put out as much as 19.8VDC. A "12VAC" transformer is going to rectify at 17VDC. For a 14VDC output you'll need 9.9VAC. Harder to find such an animal. Winding your own might be easier, but you have to understand a lot more about transformers than what I know. All I can say is that 14VAC is an RMS value (Root Mean Squared). That is to say it's "Effective" voltage because when the wave form goes higher than its RMS value the voltage is approaching zero current. I didn't say Zero current I said approaching. But when rectified and filtered via a capacitor or a battery it will see 1.414 times higher voltage than the RMS value.

Cheap car battery chargers like my father had used a 10 volt transformer and a full wave rectifier (four diodes). For now I'll assume you know the difference between full wave and half wave rectification. If you don't - just ask. It's a simple enough answer.

#### MisterBill2

Joined Jan 23, 2018
12,399
To charge an automotive sized lead-acid battery requires a voltage higher than even the high terminal voltage. Must consumer-grade battery chargers have a higher output than 14.4 volts, but it drops down to what the battery needs to charge. And if you want anything better than the slowest possible float charge you will indeed need a higher voltage capability.
Rewinding a transformer from a scrapped microwave oven is a possibility, but the 8 transformers that I have seen all had the laminations welded, which prevents disassembly without some serious metal removal, So you wind up being in the position of threading the wire through for each turn,The big risk then is scratching the wire's insulation. So rewinding like that will be very tedious. I am aware that some folks have done it, but that does not mean that it is easy.
And a microwave oven power transformer is not a constant duty device in most models nd so overheating may be a problem.

#### LowQCab

Joined Nov 6, 2012
2,170
First, let's establish why You think You need a ~20-Amp Battery-Charger.
What exact problem are You trying to solve ?

3-MOTs can be made into a top-notch High-Current-Charger,
but the project may be a complete waste of time and effort if You'll never actually need all that Current.

~20-Amps will cost You around ~$100.oo in parts, ( not including the MOTs ), so it may be much more cost-effective to simply purchase a ~5-Amp-Chinese-Charger at Walmart. You can build a precision Battery-Maintainer / Trickle-Charger for about ~$15.oo,
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#### MisterBill2

Joined Jan 23, 2018
12,399
The big benefit of the high power charger is when the battery is down and you are in a hurry and there is nobody to jump start with. 20 minutes with a five mp charger or 5 minutes with 20 mp charger.
Of course, with a bit more materials it can also be a 20 amp power supply. That can be handy some times.
I was once given a 5 amp charger that did not work. A thin wire to one of the clips had broken inside the insulation was the problem. Power wasted heating the cable is just plain wasted.
I replaced that skinny charging cord and then replaced the selenium rectifiers with a silicon half bridge pair. Now I can charge at over 7 amps in the winter, possibly more if I add a mains powered fan for the transformer..

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#### k1ng 1337

Joined Sep 11, 2020
584
To charge an automotive sized lead-acid battery requires a voltage higher than even the high terminal voltage. Must consumer-grade battery chargers have a higher output than 14.4 volts, but it drops down to what the battery needs to charge. And if you want anything better than the slowest possible float charge you will indeed need a higher voltage capability.
Rewinding a transformer from a scrapped microwave oven is a possibility, but the 8 transformers that I have seen all had the laminations welded, which prevents disassembly without some serious metal removal, So you wind up being in the position of threading the wire through for each turn,The big risk then is scratching the wire's insulation. So rewinding like that will be very tedious. I am aware that some folks have done it, but that does not mean that it is easy.
And a microwave oven power transformer is not a constant duty device in most models nd so overheating may be a problem.
When you say "constant duty device", is this synonymous to saying a DC current? Or is there a explicit difference between 100% duty and direct current? This may be a silly question although I sometimes come across the word usage such as yours where I would expect "direct current" for simplicity.

#### LowQCab

Joined Nov 6, 2012
2,170
"Continuous-Duty" is the proper term for a device rated at or above 100% "Duty-Cycle".

MOT-Transformers are infamous for scrimping on the expensive Copper,
this causes them to draw excessive Idling-Current,
which can then cause them to easily over-heat in a "Continuous-Duty" application.

For this reason, when using MOTs, it's a good idea to use two of them with the
2 Primary-Windings connected in series to reduce the Idling-Current.

Very few Hobbyists actually use proper enameled "Magnet-Wire" to make their new Secondary-Windings.
Standard THHN Building-Wire has a very tough Nylon-Jacketed-Insulation and
seldom causes any problems with the sharp edges of the Laminations.
A Fine-File easily knocks-off the sharp edges.
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#### MisterBill2

Joined Jan 23, 2018
12,399
When you say "constant duty device", is this synonymous to saying a DC current? Or is there a explicit difference between 100% duty and direct current? This may be a silly question although I sometimes come across the word usage such as yours where I would expect "direct current" for simplicity.
There is no relationship in the meanings I use for constant duty or Direct current. Constant Duty means continuous operation for an extended time, such as a fan that you switch on and have running all day. Probably I should have used continuous duty instead.
The opposite is "intermitent duty", such as a cheap hoist I have seen advertised: Run for one minute, let it cool for five minutes. A garage door opener motor would be an intermitent duty motor, as is the starter motor for an engine.

#### bassbindevil

Joined Jan 23, 2014
457
There's lots of old battery chargers out there that could be refurbished or updated with SCRs and a microcontroller. Try visiting some automotive-themed (or old tractor) swap meets (arrive very early for the best deals and to have enough time if it's a big one). Or visit a lot of yard sales, but that's less of a sure thing. Or 2nd hand and pawn shops, but remember that old selenium rectifiers are likely to be faulty, so don't overpay.
Junked 120V welding machines could be a source of a good transformer: I once saw a gutted MIG welder by the side of the road (just the transformer left). Since a welding arc is around 22V, you'll need to remove turns from the transformer, and in the case of a stick welder, there might be an inductor winding to bypass.

#### MisterBill2

Joined Jan 23, 2018
12,399
A battery charger does not need a huge transformer. Ten amps is good for anything except jump starting. If the transformer weighs two pounds that should be big enough, never go over 5 pounds until you understand fully what you are doing.

And always replace the rectifiers with silicon diodes of suitable rating.
And do not overlook the charging cord, It should be #16 size wire , and the connections must be in good condition.

#### Rufus Chucklebutty

Joined Mar 23, 2019
40
I have two transformers rated at 8 amps, can I wire both rectifiers into one pair of charging cables like this diagram?

#### Tonyr1084

Joined Sep 24, 2015
6,881
Like what diagram?

#### MisterBill2

Joined Jan 23, 2018
12,399
I have two transformers rated at 8 amps, can I wire both rectifiers into one pair of charging cables like this diagram?
The transformers are rated at 8 amps, but we need to know the voltage. Certainly two chargers can be connected in parallel, and you gain charging rate. It might not work with two regulated power supplies in parallel, unless they are intended for parallel operation.