Post by papaof2 on Jan 22, 2021 4:26:34 GMT -6
Easy question if it's a modern tool with interchangeable batteries, but what about that old cordless vacuum or work light with the wired-in batteries?
I'm rebuilding an ancient Shark Cordless Hand Vacuum, a model SV748 which came with 9 sub-C NiMH cells for a 10.8 volt battery. Maximum charge voltage for an NiMH cell is 1.4 volts so the pack's voltage when fully charged is 12.6 volts.
The new battery pack will be six cells of a nine cell laptop battery which was DOA - the Dell laptop went into "flashing orange" on the battery LED when that battery was installed which indicates a defective battery. I contacted the vendor who asked if a replacement battery would be OK. I said yes and the replacement battery was here in a week or so.
I disassembled the DOA battery yesterday and found one dead cell - a bit rusty where it had leaked around the positive terminal - so it died after the battery was assembled and tested or it would not have passed any level of testing. The battery pack is 3s3p lithium ion so that dead cell took the two in parallel with it to near-zero volts (0.23 when I measured the cells). The two possibly good cells appear to be recoverable, as a slow charge at 20ma, then 60ma brought the cell voltage above 2 volts and a 300ma charge got it above 3 volts for both cells. Most "smart" lithum chargers will NOT try to charge a cell that's below 3 volts (exactly how my smart charger handled these cells) so you need a separate source with adjustable voltage and current limits to attempt recovery of the cells.
Why did I mention the voltage of the NiMH cells when I plan to replace them with lithium ion cells? Because the numbers work out almost the same. Three lithium cells fully charged at 4.2 volts are 12.6 volts so the original charger will work (with an added BMS board to handle cell balance, over/under voltage of the lithium cells and excessive charge/discharge). Three lithium cells at their nominal 3.7 volts are 11.1 volts which is very close to the NiMH pack's nominal 10.8 volts. The 3s2p lithium pack will have almost double the amp hour (AH) capacity of the original NiMH pack (original cells were 1500mAH; the lithiums are testing out around 1600mAH and will be 2 in parallel) - and it weighs less. The cordless hand vac should be good to run the powered brush attachment on all 15 steps to the upper level of the house - and be lighter than the corded hand vac we've been using for that.
Total cost? About $8 for the 20 amp BMS board plus my time checking lithium cells and then assembling them into a pack which will fit in the space of original battery pack in the vac - easy enough to do when the lithium pack is smaller than the NiMH pack. Oh yes - also need some Captan tape to insulate all those battery terminals when they're stuck together as a pack, but I have a roll of that - somewhere...
Update: Both of the cells which were in parallel with the dead cell were recovered. One at just over 1600mAH, the other at just over 1500mAH. The other six cells are all well over 1600mAH so the new battery pack will be well over double the original NiMH pack's 1500mAH rating.
After a 24 hour rest for the lithium cells, I'll run one more discharge/charge cycle to ensure they are all close to the same capacity before welding the cells into a 3200mAH 3s2p pack. The BMS will take care of any small differences in the cells over time.
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The cells are finally together and the vaccum motor runs with great vigor ;-) Much greater vigor than for many months. The acid test was having the better half vacuum the carpet on the stairs to the upper level using the powered brush adapter (yet another power-hungry motor). The vacuum managed the carpet and the bare tread on either side of the carpet all the way down (clean from the top down on stairs so any dirt that gets from one step to another can be picked up from the step below) and I couldn't tell a difference in the vacuum motor speed when it reached the last step - as it should be, as the new battery pack is twice the capacity of the original.
There is a slight "gotcha" - the BMS has a 20 amp current limit and it seems the vacuum motor can have a start current higher than that if it stops at the right (wrong?) position. A couple of on/off cycles clears that and the vacuum works fine but I have a 40 amp BMS on order to resolve that ($5 via slow boat from China - maybe April or May).
Not a bad fix for $8 (for now; plus $5 for future upgrade) for the BMS plus miscellaneous bits of wire, solder and Captan tape. Beats $50-$60 for 9 new NiMH cells to build a new pack just like the original. If I need to rebuild the lithium pack in the future, I have a couple of "dead" 40 volt Ryobi battery packs which have a dozen or more cells - most of which can be salvaged - and a new pack only needs 6 cells.
Maybe it's time to build some lithium packs for the "12 volt" Craftsman cordless drill that used NiMH cells?
I'm rebuilding an ancient Shark Cordless Hand Vacuum, a model SV748 which came with 9 sub-C NiMH cells for a 10.8 volt battery. Maximum charge voltage for an NiMH cell is 1.4 volts so the pack's voltage when fully charged is 12.6 volts.
The new battery pack will be six cells of a nine cell laptop battery which was DOA - the Dell laptop went into "flashing orange" on the battery LED when that battery was installed which indicates a defective battery. I contacted the vendor who asked if a replacement battery would be OK. I said yes and the replacement battery was here in a week or so.
I disassembled the DOA battery yesterday and found one dead cell - a bit rusty where it had leaked around the positive terminal - so it died after the battery was assembled and tested or it would not have passed any level of testing. The battery pack is 3s3p lithium ion so that dead cell took the two in parallel with it to near-zero volts (0.23 when I measured the cells). The two possibly good cells appear to be recoverable, as a slow charge at 20ma, then 60ma brought the cell voltage above 2 volts and a 300ma charge got it above 3 volts for both cells. Most "smart" lithum chargers will NOT try to charge a cell that's below 3 volts (exactly how my smart charger handled these cells) so you need a separate source with adjustable voltage and current limits to attempt recovery of the cells.
Why did I mention the voltage of the NiMH cells when I plan to replace them with lithium ion cells? Because the numbers work out almost the same. Three lithium cells fully charged at 4.2 volts are 12.6 volts so the original charger will work (with an added BMS board to handle cell balance, over/under voltage of the lithium cells and excessive charge/discharge). Three lithium cells at their nominal 3.7 volts are 11.1 volts which is very close to the NiMH pack's nominal 10.8 volts. The 3s2p lithium pack will have almost double the amp hour (AH) capacity of the original NiMH pack (original cells were 1500mAH; the lithiums are testing out around 1600mAH and will be 2 in parallel) - and it weighs less. The cordless hand vac should be good to run the powered brush attachment on all 15 steps to the upper level of the house - and be lighter than the corded hand vac we've been using for that.
Total cost? About $8 for the 20 amp BMS board plus my time checking lithium cells and then assembling them into a pack which will fit in the space of original battery pack in the vac - easy enough to do when the lithium pack is smaller than the NiMH pack. Oh yes - also need some Captan tape to insulate all those battery terminals when they're stuck together as a pack, but I have a roll of that - somewhere...
Update: Both of the cells which were in parallel with the dead cell were recovered. One at just over 1600mAH, the other at just over 1500mAH. The other six cells are all well over 1600mAH so the new battery pack will be well over double the original NiMH pack's 1500mAH rating.
After a 24 hour rest for the lithium cells, I'll run one more discharge/charge cycle to ensure they are all close to the same capacity before welding the cells into a 3200mAH 3s2p pack. The BMS will take care of any small differences in the cells over time.
---
The cells are finally together and the vaccum motor runs with great vigor ;-) Much greater vigor than for many months. The acid test was having the better half vacuum the carpet on the stairs to the upper level using the powered brush adapter (yet another power-hungry motor). The vacuum managed the carpet and the bare tread on either side of the carpet all the way down (clean from the top down on stairs so any dirt that gets from one step to another can be picked up from the step below) and I couldn't tell a difference in the vacuum motor speed when it reached the last step - as it should be, as the new battery pack is twice the capacity of the original.
There is a slight "gotcha" - the BMS has a 20 amp current limit and it seems the vacuum motor can have a start current higher than that if it stops at the right (wrong?) position. A couple of on/off cycles clears that and the vacuum works fine but I have a 40 amp BMS on order to resolve that ($5 via slow boat from China - maybe April or May).
Not a bad fix for $8 (for now; plus $5 for future upgrade) for the BMS plus miscellaneous bits of wire, solder and Captan tape. Beats $50-$60 for 9 new NiMH cells to build a new pack just like the original. If I need to rebuild the lithium pack in the future, I have a couple of "dead" 40 volt Ryobi battery packs which have a dozen or more cells - most of which can be salvaged - and a new pack only needs 6 cells.
Maybe it's time to build some lithium packs for the "12 volt" Craftsman cordless drill that used NiMH cells?