# after automotive lead acid battery charger circuit

#### turbo1600

Joined Jan 12, 2016
7
i have a transformer rated at about 10A, anybody have a simple type charger circuit for auto batteries ? i have a lm395k if thats any help.

#### ericgibbs

Joined Jan 29, 2010
12,523
hi 1600,
What is the voltage of the transformer.?
E

#### turbo1600

Joined Jan 12, 2016
7
hi 1600,
What is the voltage of the transformer.?
E
hi, after rectifier its about 18-19vdc

#### Alec_t

Joined Sep 17, 2013
12,005
I have an ancient battery charger with a similar output voltage from the rectifier. The only things between the rectifier and the battery are an ammeter and a length of resistance wire. One of these days I must get round to adding some smarts to the charger .

#### crutschow

Joined Mar 14, 2008
27,010
If by simple, you mean a dumb charger, than you could just use a full-wave bridge rectifier and some resistance to limit the maximum charging current.
It also needs a fuse or circuit breaker to protect against accidental shorting or reverse connection of the output.

Note that this simple configuration will likely overcharge the battery if the charger is left connected past the battery fully-charged point.

#### dl324

Joined Mar 30, 2015
12,798
i have a transformer rated at about 10A, anybody have a simple type charger circuit for auto batteries ? i have a lm395k if thats any help.

#### turbo1600

Joined Jan 12, 2016
7
hi, that looks more like it, thanks, but how can it be 5amp if d3 is rated at only 3A.
i have a transformer rated at about 10A, anybody have a simple type charger circuit for auto batteries ? i have a lm395k if thats any help.

mmmmok.. looks like i screwed up , i meant a LM396K voltage regulator, NOT lm395k.. sorry..

#### crutschow

Joined Mar 14, 2008
27,010
but how can it be 5amp if d3 is rated at only 3A.
Yes, D3 should be rated for at least 125% of the maximum charging current.
i meant a LM396K voltage regulator,
That can be substituted for the LM338 with no other circuit changes.

Below is the LTspice simulation of a simple battery charger circuit using the LM338 (or LM396) with an added current limiter.

Transistor Q1 turns on to reduce the output voltage, and thus the current, when the voltage across shunt resistor R4, due to the charging current, ≈ 0.65V at typical charging voltages.
The current limit is thus about 0.65V/R4.

The output voltage charging limit is determined by R1 (gives a value of about 14V for the value shown).
D1 prevents damage to the circuit if the battery is accidentally reverse connected (output pulled negative).
D2 prevents damage if the battery is connected without input power.

The simulation shows the battery voltage (simulated by a large capacitor) going from 10V to 14V (blue trace) with a charging current of around 4.5A (yellow trace).
Note the dissipation of the regulator (red trace) shows that it needs to be on a large heat-sink.

Edit: I just noticed that the LM396 has a maximum in-out voltage rating of 20V, so could fail for a reverse battery connection, which could put up to about 30V across it.
You could protect against that by putting a power diode from output to ground (anode to ground) with a fuse between that and the output.
That would blow the fuse for a reverse connection.

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