Reviving the 36/40 Volt Battery for a Ryobi Leaf Vacuum

At a dinner party a few weeks ago we got into a conversation about gardening. This started an argument between a couple about their cordless Leaf Mulcher. One accused the other of letting the battery go flat, so it no longer worked. The other asked why didn't they buy a corded model in the first place, as they only had a small balcony garden. Being handy with electronics I offered to help by reviving their battery or finding a replacement.

I understood some of the heat in argument, when I found this was a relatively expensive Ryobi Leaf Vacuum, with a replacement 5 AH battery costing several hundred dollars in Australia. 

There are numerous web sites which explain these batteries can fall into a sleep mode if left uncharged, but can be revived. However, lithium batteries are made with very reactive materials and if mishandled can explode and burn vigorously, emitting toxic gas. So I was reluctant to try charging the battery with other than the supplied charger. 

Before I could start work on the cordless unit, I happened to find a second hand Ryobi Corded Leaf Blower & Vacuum, at Salvos Stores Leichhardt, for $50, which the couple were very happy with. But I thought I might as well get the cordless unit to work. 

To provide the 40 volts needed to charge the battery, I connected three computer power supplies in series. The Ryobi batteries are sold in Australia as being 36 Volt, but in North America as 40 Volt. The supplies could produce a maximum of 1 Amp, thus limiting the power to the battery.

The + and - terminals on the battery read 20 Volts with a voltmeter. I connected these to the 40 volt power supply, but read only a few microamps charge. There are two other terminals on the battery marked T1 and T2, which I assumed were for temperature monitoring. When I checked with the voltmeter between the + and one of the T  terminals, the current suddenly jumped to 700 mA for a few seconds, then dropped to zero, then jumped again, while the lowest of the charge indicator lights on the battery flashed. After a half an hour the battery had enough charge to work in the regular charger.

I dropped the unit off at the Bower Reuse and Repair Centre,. I noticed they had a few other blowers and leaf mulchers second hand as well.

Please note that this is not an repair recommended for the inexperienced. Do not leave the battery charging unattended.