Mnem, is this the kind of cheap Chineseum BMS 8S board that could be used in the Tek 422 scope & why only 7sections on the board? Would have been nicer to find something in the UK, but all those seem to be for larger solar/battery charging.
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Original battery pack was 24V Ni-Cd D cells, I'm thinking of using LiFePo4 cells, I would need to modify the existing charger & low battery circuits on the scope.
David
It's an active balancing circuit; instead of bleeder resistors it shunts current from the highest cell to the lowest. It only needs active elements on B1-B7 to do so.
LiFePo4 tends to not like high current draw; you want to design around a 1-3C discharge rate. I see a 3A fuse in the battery circuit so pretty much any 18650-32650 Lixx cell of 3000mAH capacity or more should be a good match. Obviously you're going to need to make sure your BMS is either LiFePo4 firmware or programmable for LiFePo4. 20S Nixx application is a great candidate for a 8S LiFePo4 pack upgrade.
I suggest something like these, if you can get them where you are and have room for 2-7mm longer cells (32mm x 65-70mm), as they are sanely priced and have a nice 3C discharge rating, so you're definitely not going to be beating them very hard, and they should give you many many cycles. They are very close to the dimensions of a D-cell, which comes in at approx 34mm x 63mm:
https://batteryhookup.com/products/100-brand-new-lifep04-32650-3-2v-6000mah-cells
If your original pack was 10x2 configuration, that would be somewhere around 340mm x 68mm x64mm tall. Turned sideways in a 4x2 configuration, these would be ~270-280mm x32mm, so you could double-stack for 8S2P config at 270-280mm x64mm x 64mm "tall". This would still leave ~ 64mm x 64mm x 60-65mm deep at the end of the pack for the BMS board. Typical for these cells is 3.8-6AH capacity, so you'd be right around 7.6-12AH vs 8-10AH capacity for a full D-cell NiMH/NiCd battery.
Do you know what the actual float voltage and charge current of the existing charge circuitry is? From looking at the circuit... it appears ~27.1V less ~.6Vfd of that diode, so 26.5V / 8 =3.31V/cell... Hmmm. You really need to be charging at 3.4-3.5V/cell minimum, even if short-cycling to prolong battery life.
What's the Low-Batt light threshold? I see 11.5-35V operating voltage; if that reflects LVC (0.58V/Cell for the NiCd does seem a wee bit low) but the light comes on at a more conservative 1V/cell or 20V, you might actually be right in the ballpark at 2.5V/cell or 20V for the LiFePo4 pack. Then you'd just let the LVC in the BMS stop you from over-discharging the pack.
mnem
*juicy*
Thanks for that Mnem, it's helped a lot.
![ThumbsUp :-+](https://www.eevblog.com/forum/Smileys/default/icon_smile_thumbsup.gif)
I've had a read through relevant sections of the manual for the later version with no nuvistors, as mine is an older one with the three nuvistors, the quoted figures will be slightly lower.
Power consumption on DC is 26W max, current = 26 divided by the input voltage (11.5V to 35V) = 2.26A to 0.74A. 1.09A at 24V, will be higher with older version.
It takes 16 hours to charge the 20 series D NiCds, 5 hours operating time, 30 mins less with scale illumination fully on.
Charge rate is 400mA on mains charging only, 30mA when operating at the same time, no charging on external DC, the thermal cut-out also cuts charge rate to 30mA.
The low voltage indicator circuit flashes the power light when battery voltage is below 22V.
I've done some tests on mine earlier today, it appears the charging circuit is a current limiter only, open circuit voltage goes up to 62.5V, I also managed to smoke the 180Ω resistor for the 30mA trickle charge during testing, more on that below.
![Evil >:D](https://www.eevblog.com/forum/Smileys/default/evil.gif)
First I wanted to test the charger was functioning correctly at the two charge rates, a 24V 12W lamp was used as a load, the fast charge rate was 400mA and crept up & stabilised at approx 413mA once run for a while, voltage across the load was approx 18V. The slow rate (30mA) was then selected, current was a bit higher than the book stated at 48mA after running for a while.
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Then I stupidly decided to find out if it would overcharge the potential replacement 8S LiFePo4 batteries, I swapped the lamp for a 240V 150W one, the current at fast charge was 272mA and 47.4V was measured at the battery connector, looks like I will need to add OVP to the replacement battery pack.
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The slow charge was selected again and I got a smell of something cooking, I quickly turned it off, separating the PSU I found the 180Ω carbon comp resistor that is added in series for the slow charge rate had got a bit burnt, it now measured 76Ω with the charred casing.
![Broken! :-BROKE](https://www.eevblog.com/forum/Smileys/default/smiliey_scope_broke.gif)
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I'm not sure what happened, maybe the test exceeded it's power rating, or it was already on it's way out? It took a while to locate my spares, I could only find a slightly larger metal film type.
In it went & retesting with the 24V lamp, it appeared no other damage was caused.
![Phew :phew:](https://www.eevblog.com/forum/Smileys/default/phew.gif)
The next test was to run it on DC and find measure the current draw at various voltages, with the scale lighting on & off, intensity was set at a sensible level, with the internal setup signal selected for both inputs.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1460338)
Measurements were taken with two DMMs, as the Sorensen PSU meters need calibrating.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1460332)
Looks like the maximum current for the 8S LiFePo4 would be 1.2A, I probably don't need to go above 6AH capacity to meet the run time of the NiCd pack, not sure I need more run time, as it would increase both the costs of the replacement battery pack & charge time.
Last test was done at the same time as the current measurement, the low voltage indicator circuit is currently set at 21.8V, it latches the power lamp in a flashing state.
Need to work on the case next, the battery pack would have been attached inside, an old D cell is shown for scale, measurements have not yet been taken.
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David