DISCLAIMER: Please note that the following experiments were conducted under controlled conditions. No responsibility will be taken for any outcomes including damages incurred from the use, misuse or inability to use the information that follows. You are responsible for any actions that you take. Your mileage may vary!
The humble Ni-MH battery has been with us for many years and is now the less-preferred alternative, with Li-Ion and Li-Poly batteries being nearly ubiquitous. However, everyone knows that many Lithium-based cell chemistries are somewhat volatile, with potential safety issues especially if placed under stress or approaching end of life. On the other hand, Ni-MH cells while more fragile than the Ni-Cd cells they replaced, are not as volatile. So perhaps the two might be able to cross paths in an unexpected way?
The Motivation for Constant Voltage?
The motivation for exploring constant voltage charging regimes lies in the fact that Li-Ion/Li-Poly cells normally use a current-limited constant voltage charging regime with a termination current level. For traditional 3.6/3.7V Li-Ion cells, the maximum charge voltage is normally specified as 4.20V. Newer, specially treated cells, are capable of tolerating higher voltages for slightly improved capacities, with the latest 3.85V nominal cells having a charge termination voltage of 4.40V which would otherwise potentially cause catastrophic overcharging of regular 3.7V cells.
Unfortunately, as the Li-Ion/Li-Poly batteries in certain devices start to fail, finding quality replacement batteries is difficult. In the case of mobile phones using exotic 3.85V varieties, the cells are virtually unattainable, making rebuilding the pack an impossible endeavour. Sometimes there are “so called” genuine replacement cells which have all the hallmarks of being counterfeits – same serial numbers and dates, spelling mistakes and omissions. In the past, I’ve purchased a few only to realise even worse performance compared to the failing battery it replaced, along with the full risk that opening the device entails.
Unfortunately, as the battery is an integral part of the power regulation design of these mobile-phones which are kept on a constant “micro-cycling” role, it is not possible to operate the phone from USB power alone. To extend the lifetime of such end-of-life devices would require substituting the battery somehow – in my mind, perhaps a very large capacitor could be used, but then I thought why not try Ni-MH cells?
Looking around, it seems the general advice is never to attempt recharging a different chemistry than intended on a given charger. While this is safe advice to give, it doesn’t necessarily tell the whole story. I wanted to know if there were cases where this could be possible even if it doesn’t realise the full cell capacity. For a phone that might be stationary, hooked up to USB all the time as an LTE modem, it really wouldn’t matter.
Constant Voltage Charging Considerations
In theory, with 4.20V termination voltage, using three cells would result in 1.40V on each cell (assuming they are perfectly balanced), and with the higher 4.40V termination voltage, this rises to 1.47V. This didn’t seem too far from the nominal 1.20V rating of the batteries and I know chargers frequently apply up to 1.65V or so to the battery to charge them quickly – due to internal resistance and charging inefficiency, you often will need to put power in at a higher voltage to get it to flow back into the cell.
The problem is that Ni-MH cells are supposed to be charged with a constant current, but monitored for a negative change in voltage or temperature rise that signals the end of charging. As a result, they are not really intended to be on permanent float (many specify very low currents of C/40 to C/100 for float service, others specify periodic top-up once a lower voltage threshold is reached). How they behave with a constant voltage applied is not really specified and will probably come down to the cell and its internal resistances. In the worst case, I would expect the cell to continue drawing current at a rate that is high enough to cause it to heat-up. If it heats up sufficiently and builds up enough pressure, it may open the over-pressure relief vent where the cell will vent and potentially dry up, losing capacity. If abused at extreme rates, explosion of the cell is possible, and some fire could be possible.
I also have not worried about the difference in discharge characteristics between the two chemistries, which probably deserves some mention. As three Ni-MH cells have a summed nominal voltage of 3.6V, it is likely to register about half-full for a 3.6V lithium based system, but for 3.85V systems, it may register significantly below-half. As the Ni-MH cells may have a higher internal resistance, it may cause the voltage to drop enough to trigger low-voltage shutdown on some devices depending on their current demands. As a result, even if they are capable of handling constant voltage charging, they may not be a suitable replacement for this (and other reasons). At the least, the battery capacity indication would be wildly inaccurate.
Of course, the voltages calculated earlier are based on cells being in perfect balance, which may be possible using carefully chosen cells at the beginning of life. But as they age, the cells may lose capacity at different rates and without any circuitry that balances the cell voltages during charging, they can grow out of balance resulting in some cells receiving overcharge while others are being undercharged. I won’t consider the consequences of imbalance too thoroughly, but instead, will look at single cell behaviour.
To better understand how Ni-MH cells behave in constant voltage charging, I chose a 2013 Sanyo Eneloop AA cell of 2000mAh, Made in Japan. Before each test, the cell was fully discharged using a B&K Precision Model 8600 DC Electronic Load at 1A until the remaining cell voltage was nearly zero (such that the load was incapable of sustaining a 1A load). The cell was left to return to room temperature prior to charging. The cell was then connected to my Keysight E36103A power supply to be charged at a constant voltage, with a 2A current limit. Simultaneously, a K-type thermocouple was mounted to the body of the cell using electrical tape and monitored by a Keithley Model 2110 5.5-digit digital multimeter. A variety of constant voltages were tested to determine the charge characteristics over a relatively long period of up to 23 hours at normal room temperatures of about 21-23 degrees Celsius.
It is important to note that the results presented are for a single cell, of a single trial and may not apply to all makes of Ni-MH cells depending on their internal resistances and cell capacity. However, it should prove to be indicative as to whether there is a potential for constant voltage charging regime to be functional.
Constant Voltage at 1.500V
As the even voltage split from a 4.40V terminating charge across three cells is 1.47V, I decided to start my experiments with a supply of 1.500V. Interestingly, while the current initially peaked, it fell off to around 1A where it lingered for a couple of hours and then fell as the cell became full. The temperature did increase slightly to around 31 degrees C during this period, but once charged, the temperature fell to slightly above room temperature (likely as the cell was dissipating the excess energy as in a trickle charge). At this voltage, the current remained within 40-60mA after it had “finished” charging, with a slight temperature dependence. This seems to be low enough that a Li-Ion charger circuit may terminate the charge automatically – but also low enough to be about 0.025C so within the ~0.03C which is considered safe for float charging the cells at.
As a result, this gives some promising encouragement that constant voltage charging may be possible with Ni-MH cells, providing their internal resistances mean that the cell limits its own charge current at the supplied voltage.
Constant Voltage at 1.600V
Buoyed by the success of my first test, I decided to step up the voltage to 1.600V. This would represent the result of some imbalance – say if two cells were 0.05V lower, then one cell would be subjected to 0.1V more. In this instance, the cell heated up to about 35 degrees C during charging, with a similar curve but didn’t level out at a current as low as the previous. Instead, it levelled out at closer to 320mA before slowly falling to about 120mA. This is not an ideal condition to float an Ni-MH cell and might cause it to eventually lose capacity, but is still not catastrophic in terms of current that it would cause the cell to vent or explode. The current seems to be dependent on the temperature somewhat – so perhaps at this rate, it wouldn’t be possible for the charger to auto-terminate on current, and may instead have to do on maximum charge time instead.
This test provides some necessary information – namely that the cells could probably handle a slight imbalance as they have some margin. As the cell does continue passing current once fully charged, it could potentially “even” out the imbalance becoming “top balanced” but only if the charger doesn’t terminate the charge prior to the cells attaining reasonable balance. As a result, it’s probably not a good idea to draw the cells down to flat, but the Li-Ion protection circuitry should cut out early enough that no permanent damage would be done to the cells.
Constant Voltage at 1.625V
The sharp temperature rise in the previous test is an indication that we are on the brink of the cell going into “runaway” charging, so I decided to step it up only a tiny bit to 1.625V. In this case, the runaway appeared – note that after the charging “completed” and the current dipped, it rose again accompanied by a steep rise in temperature. This rise in temperature correlated to a slight rise in current, which eventually “ran away” as they reinforced each other, hitting my 60-degree cut-off which shut down the charging process. As a result, charged at a constant voltage, it will be very sensitive to the choice of voltage and perhaps a thermal fuse may be necessary to ensure safety.
Constant Voltage at 1.700V
Just for fun, I decided to push the voltage up to 1.700V, knowing that many chargers won’t ever apply more than about 1.780V to a Ni-MH cell, just to see what would happen. Needless to say, the test only made it to around an hour and twenty minutes before it was terminated due to over-temperature. This time, we ran into the current limit initially, suggesting that about 1.63V is needed to push a full 2A into an empty cell, but once the charge completed, the temperature fell slightly before shooting up wildly, and likewise, the current did similar things. I suspect in a CV charging regime, perhaps this bump in current could be used as a way to detect end of charge as a proxy for detecting cell temperature rise. Quite fascinating indeed.
In general, Ni-MH cells are supposed to be charged in a constant current regime. However, with some care, it seems that it might be possible to have them charge on constant voltage as well, provided the right voltage is applied such that the internal resistance acts to limit the charging current to a safe value once the charge is completed. As a result of tests on a single Sanyo Eneloop AA cell, it seems that constant voltage charge up to 1.600V was “stable”, with charge at 1.500V seemingly “safe” under the test conditions at the current was limited to safe float-charge currents under regular room temperature conditions. However, above 1.625V, the cell was unstable, exhibiting thermal runaway once fully charged.
So it seems that it might be possible to use three Ni-MH cells to substitute for a Li-Ion/Li-Poly cell based on this, assuming the batteries are not running at elevated temperatures, are in reasonable balance, assuming that the faulty battery indication is no issue and assuming load currents are within the capacity for the Ni-MH cells such that their voltage does not fall too far. However, it still seems sensible to put in at least a thermal fuse to prevent catastrophic overcharging due to the voltage sensitivity of the process, and it is important to be aware that other Ni-MH cells may have different internal resistances that will result in different currents.
In the end, this is not something I can recommend to the general public, although it may still be possible to do in a workable fashion. Interestingly, observing the current falling and then rising again as a proxy for cell temperature increase could actually be a form of charge termination for a CV charging regime, if ever one was needed. Although, as the regular CC regime works well in cases of dedicated Ni-MH charging, I suppose this discovery would be of little consequence in practical terms.