Determine a suitable replacement battery cell for an obsolete product?

Question

A friend of a friend of mine has a prosthetic knee that was developed in 2010 which requires battery packs that are being phased out. He has asked me if there is a way to replace the internal batteries for these units, so I cracked one open and they seem to be an array of about 12 x 3.7v lithium ion batteries arranged in two parallel "strings", each of 6 cells in series (6S2P) (approx nominally 22V per cluster) of the following type link . These cells individually are not available for anything less than 10,000 unit purchases, which obviously is not really an option for this one off repair. I am a computer engineer with some experience in power systems but I have never worked with a lithium ion source before and am trying to be cautious about choosing an appropriate replacement battery.

I have been looking at these two batteries as replacements. Each vary a little bit from the source battery, either in size, capacity, or discharge rate. What I am curious as to is whether these variances from the original pose any significant risk? I suspect the power characteristics are close enough to provide at least ... some functionality, however the charging circuit and battery packs are pretty elaborate and I would hate to drop $150 in battery cells just to find out that they didn't work or worse... caused a safety issue.

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If it's of any interest, here is the link to the product that this comes from Power Knee

If there are resources as to answers for these questions, or links to other battery suppliers who may be able to meet my requirements, I would accept any and all feedback. Thank you in advance.

Answer 1

NB The following comments and 'recommendations' relate to a battery used with a prosthetic device. While the advice is believed good it is offered with the usual internet device disclaimers [ACNR, YMMV, IANAL, do not spindle fold bend staple punch mutilate, ...]. "Burning man" experiences are greatly to be avoided :-(.

Does "being phased out" translate to - "not being available AND no alternative being sold by manufacturer"? Is so, multiple customer action may improve chances of a better response. However, if a higher capacity battery is being offered then it MAY be a better solution.

Any cells of the LiPo / Lithium ion type that have adequate capacity would be suitable.
The originals that you cite are well specified in that data sheet. Their capacity is small by modern standards, (1320 mAh) and typical charge current of 660 mA = C/2 is modest.

The Digikey battery that you give as a possible replacement datasheet here is very slightly larger dimensionally so may or may not fit. It has substantially larger claimed capacity (2000 mAh vs 1320 mAh), and much longer claimed cycle life.
The stated normal charge rate of 0.2C/400 mA is below the original's C/2/660 mAh but well within the allowed C/1 / 1A charge rate.

Importantly when changing LiPo battery capacities in equipment - the new cell's recommended charge terminate specification is C/100 (0.01C) in standard or fast charge modes. Lower terinate currents = "harder" charge, and C/100 is a very aggressive charge specification (max energy shoe-horned into cell) and not conducive to maximum cell life, and it is likely that the existing charger is not as aggressive in its charge termination.
This means that, as long as the cells fit mechanically, then the new cells are probably suitable replacements because:

• Existing charger end point current is probably 'gentler" than allowed.
• While existing charge rate is probably higher than nominal recommended it is below the max allowed.
• The new cells are of usefully higher claimed capacity and longer claimed cycle life.

You do not say whether the original charger has cell balance capabilities. If not, the ability to occasionally check cell balance and to remedy imbalances would help lifetime.

If you can INCREASE charge termination current to C/10 or C/4 (and C/2 is better still) you will get a longer cell life at minimal loss of capacity - and still well above early capacity.

Again - all care, no responsibility. I'm happy to comment further on or offlist due to the important application.