During a discussion over a friend's flat car battery they brought out their 'car battery booster' but it soon became apparent to me that that also wasn't working....and so, like most tinkerers, engineers and makers, it ended up on my bench at home.


The unit contained a 12v 13Ah Sealed Lead Acid (SLA) battery, large diameter cables and croc clips for connecting to the discharged car battery, a voltmeter, a light and an external cigarette lighter socket. Charging the booster unit was via an underwhelming mains wall wart that was rated at 500mA and 12v DC output. When that was plugged in to the mains power and connected to the booster device the charging LED would blink briefly and go out again. Pressing the test button showed that internal SLA battery was about 12.5v. There was no difference in the voltmeter when the charging source was connected or not. Time to get the case open.


The caveat at this point, to anyone replicating such an investigation, is that whilst the charging voltage was low (~12v) and of minimal current capacity, and whilst the internal SLA battery voltage was low, that battery would be able to deliver some serious current during a short circuit. Enough current to quickly heat any metal object shorting the battery possibly causing splatters of molten metal or a nasty skin burn. So some safety spectacles are advisable along with caution when using tools inside. On with the fun part of fault finding.


Inside the unit was quite basic. Most space was taken up with the SLA battery, there was the back of a high current switch, a small single-sided PCB with about a dozen components and lots of black and red wires between the various components. (see my comment to #MakerTips - Essential Tips for Makers  regarding red-red and black to black ).

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Reverse of the PCB


Component Side of PCB - relay on Right


Inside - round device with thick cables is the output on/off

Using a DVM I measured to open circuit voltage of the battery which was about 12.5v. I connected the charger and turned on - LED on and back off again, but at the same time I heard the relay click. I wanted to test out the components individually but not with that 13Ah power source connected just in case. So I unbolted that and got it out of the way - paying close attention that my spanner would not short terminals (even if dropped).


Once the SLA battery was out I traced and drew out the circuit, which I believe looked like this below.

Battery Booster

All the components seemed to check out well although I didn't get to measure the zener voltage as my DVM only has an open source voltage of 2.5v (so it was something greater than that) - but the forward voltage checked out OK.


Soon I was looking at the relay, which appeared to be an open framed micro style however when I came to reassemble the unit I realised that the plastic cover had been pulled off and remained attached to the booster's casing. That was a benefit to me though as I could probe the relay. The relay was normally closed but that didn't hold true when tested with the DVM. Pressing the mechanism caused the relay to sporadically short. I assumed there was some dirt or corrosion in the contact so made a thin paper strip, soaked it in methylated spirits and pulled it through the contacts several times. This didn't resolve the issue.

Boosters Relay

Using an eyeglass and moving the mechanism with a wooden cocktail stick I could see that the spring mechanism holding the relay in the closed position seemed weak. Another breakthrough was that it was flexible enough to allow me to pull the relay arm from under the pole, this allowed me to give it a bend upwards by holding it in some tweezers and twisting. I was able to slide it back in under the top pole. Now the relay showed a short on the DVM as expected as it was in the normally closed position.

Booster Relay - contact pulled back

[Photo above shows the relay with the contact arm extracted out and lying on top of the relay chassis...prior to being bent upwards]


After a few tests I reconnected the SLA battery and put the back of the case on. I connected the charger and on came the LED, after a few hours the battery voltage as shown on the built in voltmeter has also started to increase. Last thing to do....return the booster unit to its owner !



The relay had lost its spring force, it therefore wasn't making a reliable contact. The relay, transistor and zener are there to prevent over-voltage on the input charging from damaging the SLA battery. The two 1R resistors act as a crude current limiter. A nice little repair that should keep this going for a bit longer.


And now to explain my blog post title. I recently did a repair on a TV and called it 'No User Servicable Parts' as here so it was a natural follow on from that, kind of like a theme. Although this particular unit had no such warning. Happy mending