How to use pushed NiMh cells

Following article is based on correspondence with Mr. Juergen Bestenlehner who is very experienced with pushed cells distributed by Hopf.

Q: "What are practical experiences with pushed NiMh cells usage and how to care about them?"

The question of using pushed cells is a little different in pushed NiCd and NiMh cells. For NiCd cells will be no difference in charging, storage and using them as un-pushed cells. But there is one thing you must know about the effect of pushing. You would not get more capacity but a much lower resistance what gives the cells a higher voltage when using them. The result will be that you can flight a model that needs may be 22 normal cells, now with only 20 pushed cells, to get the same performance. Or you put 22 pushed cells in and get a higher current.

For NiMh cells like the TS 1700AUP and also the GP3300 are more things to look after. When pushing NiMh cells the resistance can get down to the half at some types. They should be charged the first time at 1/10C for 14-16 hours, then discharge to 0,9V/cell, and then you can charge them fast. But do not use any automatic-program on your charger especially Schulze-636+, because they recognize the very low resistance of the cells and go up to the highest charging current, sometimes 8A. But the NiMh cells do not like this high current. So charge them with constant amps at max.2C or a bit lower - the TS1700AUP at 3-3,5 A and the 3000-3300 at 5-5,5A. They should get 40-45° temp when they are full charged.

And now comes the big difference to the NiCd cells. Use the cells directly after charging until they are warm and you get the best results. You will also feel that the cells come out of the plane after flying with lower temp as you put them in. But you should not discharge the cells after flying by using the motor, until there is no more rpm. This will make the cells very hot and can destroy them. Just let them get cool after flying and charge next time. May be you can wait 30-50 min before next charging.

For storage the cells discharge them to 0,9V/cell. If you store them for a week or longer charge a little 300-400mAh and look sometimes for the voltage, a 10cell pack should have his normal voltage at 12V or higher, if lower charge a bit. You can also store full charged. When next use discharge once more to 0,9 V/cell a few hours before and charge fast, just before flying.

Pro skladování vybíjejte články na napětí 0,9 V na článek. Pokud mají být uloženy týden a déle, nabijte tak 300-400 mAh a občas zkontrolujte napětí, články by měly mít nominální napětí nebo vyšší. Pokud by napětí pokleslo, dobíjejte. Také je můžete skladovat plně nabité. Pár hodin před dalším použitím je vybijte na 0,9 V a pak můžete rychle nabíjet těsně před letem.

Q: "And what about the durability of pushed cells? People say the effect is lost after one year of usage."

I think you just have to take a little bit care on these cells but I also think that it does not matter if you use normal or pushed cells, both need some care if you want to use them for years. Just the NiMh cells need more care (see "charge and discharge"). In our club must be more than 300-400 pushed cells from Hopf and some of these cells (10 and 14 cell-packs) especially pushed Sanyo-RC2400 have lost some of their quality after 1,5 - 2 years but I have to say that those cells were used at over 50 A and also in hotliners with up to 80 A. If you will try this with un-pushed cells they will not function for this long time because of their high resistance. It is also true and there will be no discussion we do not get the 1000 cycles. If you read the small letters what are printed on some cells we should charge them slow for 14 hours or fast in may be 3-5 hours (I want to see the faces at Sanyo if we ask for warranty when we tell them what we have done with their cells). I think in our case of using cells it will be O.K. when we get up to 250 maybe some more or less cycles.

In fact, some cells may loose the effect of pushing after on year or more (depends in using them). One of the effects of pushing is to get a lower resistance (Ri) so it just will get higher like un-pushed cells - they won´t work for very high current. But the cells are not destroyed if this happens. Other effect is getting better contact from chemic to battery poles and this is not lost. Loosing complete contact (cell destroyed) to the battery poles is the effect of using not professionally equipment when pushing cells. Another point to get a longer durability is matching (selecting) cells by resistance (Ri). This procedure is also done by Hopf to get cells of same Ri in packs or delivery when ordering single cells.

Karl Heinz Hopf suggests get contact to the race boat scene in your country, they also can tell you something about pushed cells. Some of that people also have pushed cells from Hopf.