Sealed Lead-acid Battery Charge Circuit for Jump Starter

On the left is a "Jump Starter" which contains a sealed lead-acid battery and originally had two big cables with crocodile clips to help start car engines when the car battery is flat. I never used it for that, I took off the big leads and added a fused cable to power my FT847 radio for portable operation. Not only does the case make the battery safe and easy to carry, but it also has a built in volt meter and an emergency lamp. On the right hand side there is a "cigarette lighter" type accessory socket to power other items or smaller radios.

The Jump Starter came with a mains adaptor to charge the battery via a power socket underneath the meter.

This is the weak-point of the system, because the adaptor is unregulated and when the battery is fully charged, the voltage reaches over 20 Volts. This has almost certainly caused the premature failure of the original battery, because lead-acid batteries should not be charged to a voltage higher than 14.4 Volts.
If the Voltage is regulated to 13.8V then the battery can be "float-charged" continuously.

There is plenty of room inside the box, and I have wanted to install a Voltage regulator for a long time. But I discovered that I could buy (very cheap!) a little regulator board from ebay.
https://www.ebay.co.uk/itm/LM317-Adjustable-Converter-Voltage-Regulator-Module-Variable-Power-Supply-DC-DC/263896453817

The module came with a piddly little heatsink, but, as the regulator is close to the edge of the board, it is easy to add extra aluminium.

There are just four connections: input +/- (on the left) and output +/- (on the right). The voltage can be adjusted with the multi-turn preset at the bottom of the picture. It is wise to twiddle the adjustment screw to get 13.8 Volts before connecting the battery.

While adding extra heatsinking, I also added a smear of heatsink compound - as there was none on the device.

Now purists will complain that there should really be a current limit incorporated into the charge circuit, because a very discharged battery will draw a high current when charged. And this is true, however the existing circuit includes a one-ohm, high power resistor in series, and also the mains adaptor is rated at 500mA, and simply can't supply too much current for a lead-acid battery of this size. Whether the mains adaptor is really adequate is another matter.

Here is the circuit of the Jump Starter. The fused cable to the radio is connected straight across the battery terminals.

Hopefully the replacement battery will last longer now, although it is a smaller capacity than the original, which was claimed to be 12Ah. I say "claimed to be" as it weighed less than the 7Ah battery that I replaced it with and therefore probably had less lead in it!

Now the meter reads 13.8 Volts when the battery is charging.
This is good.

There is a data sheet for the LM317 regulator IC here:
http://www.ti.com/lit/ds/slvs044y/slvs044y.pdf

The internals are a spaghetti of wires, but it is all well insulated :-).

The LM317 with a piece of scrap aluminium as a heatsink, board is mounted top right.

If you want to be scientific, then the "waste heat" that the regulator has to get rid of is the difference between the input and output voltage multiplied by the current flowing through the regulator. Commercial heatsinks are specified with "degrees per watt" which tells you the amount that the temperature of the heatsink will rise when it is dissipating 1 Watt and can be multiplied by the wasted power to work out whether you can fry eggs on it.

Or you can leave it on for a bit and feel (carefully) whether it has got warm or not. In my case not.
73
Hugh M0WYE