Background
One of my neighbours has a cordless garden grass strimmer powered by an 18V rechargeable Ni-Cd battery. Towards the end of the summer of 2015 she reported that the strimmer was not holding a charge and seemed to operate intermittently. The strimmer is about 6 years old and does not get a lot of use. It had also been stored in a garden shed including over winters which would not have been too good for the battery. I said i would have a look at the battery to see if there was anything that could be done to repair it.
I searched the internet for a replacement battery but could not find one. I contacted the original supplier but surprise, surprise they no longer supplied spares, something they ceased to do so for this device about 12 months after it was originally purchased. It seemed such a waste to discard a perfectly working tool that simply required the battery pack to be repaired or replaced so i decided to see what could be done.
I searched the internet for a replacement battery but could not find one. I contacted the original supplier but surprise, surprise they no longer supplied spares, something they ceased to do so for this device about 12 months after it was originally purchased. It seemed such a waste to discard a perfectly working tool that simply required the battery pack to be repaired or replaced so i decided to see what could be done.
The battery simply clips off the strimmer body and is connected to a mains charger unit to recharge the battery pack.
The mains charger produces an unregulated 23V DC at its output.
In this picture the base of the bateery pack has been removed to reveal the batteries and the circuit board which controls charging of the battery.
The batteries were in generally good physical condition given that they had spent a number of winters in a damp shed. There were signs of rust on some of the inter-battery connections and the batteries themselves.
The mains charger produces an unregulated 23V DC at its output.
In this picture the base of the bateery pack has been removed to reveal the batteries and the circuit board which controls charging of the battery.
The batteries were in generally good physical condition given that they had spent a number of winters in a damp shed. There were signs of rust on some of the inter-battery connections and the batteries themselves.
Here the battery pack has been removed from the case showing its very simple construction and the charging circuit board.
The circuit board was reverse engineered to create the circuit diagram. The circuit is very simple(crude ?) and has no protection against overcharging the batteries. It is up to the user to read the label on the side of the battery pack recommending that the charge time should not exceed 3-5 hours.
While performing a couple of charge and discharge cycles on the battery pack i noticed that if the pack was moved the charging/discharging stopped.
On closer inspection i noticed that the set of welds on one battery looked to be of poor quality and when i tried to lift it with a small screw driver it simply came away.
This would explain the poor charging when the battery was moved while i was testing it and also while it was being used to power the strimmer while cutting grass.
As i could not weld the battery terminals i had to solder them. Soldering battery terminals always requires great care as excessive heat can damage the internals of the battery. the terminals were cleaned and solder applied to the battery end and the contact.
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The terminal was held in contact with the battery terminal and the soldering iron applied for as short a time as possible while the terminal was held in place with a screwdriver. This produced a good solid contact. The battery was then put through a number of charge and discharge cycles to recondition the battery pack.
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After a few charge and discharge cycles the amount of time the strimmer could be used continuously increased from 10 minutes to 20 minutes. These pictures show the battery pack out of its case while it was being tested and cycled to try to recover it from its poor condition.
As part of the charge and discharge cycle aluminium clad wire wound 25 Watt resistors as shown in the pictures were used as a dummy load.
The battery was used in the strimmer on a couple of occasions towards the end of the season and it performed well. The battery and the charger will be kept indoors over the winter to help maintain its condition so that it will provide at least another couple of years service.
As part of the charge and discharge cycle aluminium clad wire wound 25 Watt resistors as shown in the pictures were used as a dummy load.
The battery was used in the strimmer on a couple of occasions towards the end of the season and it performed well. The battery and the charger will be kept indoors over the winter to help maintain its condition so that it will provide at least another couple of years service.