Cheap NiMH - worth what you pay

[alan 0]

I bought a Hyperion Emeter 2, the one with separate datalogger and built in servo tester, on ebay. The internal 6v Nimh battery was almost dead, didn't respond to cycling so I thought to buy a Sanyo Eneloop pack, which I know to be excellent batteries, I use loads. But being a cheapskate I bought Vapextech at one third of the price - they loose about 5% of charge per day!

 

Has anyone else used this brand? do they have a good or bad reputation?

 

Just to complete the story, I did buy some Eneloop cells but wasn't paying attention and soldered two cells together. They took their revenge by melting!

[Bravedan 106]

Instants or Std?

 

Have more than 150 std Sub C 4 cell packs, some over seven years old..................been great

[Trevor 25]

I have had Vapextec Instant AA Rx and Tx packs. I have found that they last about 3 years in normal use before one or more cell starts to go and they stop giving full voltage. The instants seem to drop about 10% charge over a month if left idle after a full charge, judging by the fact they usually need about 250mah top up after this time. I have eneloops too and they seem to lose about the same, maybe a bit less but not significantly. They do need a few cycles before they start working properly.

[Martin 0]

The instants seem to drop about 10% charge over a month if left idle after a full charge, judging by the fact they usually need about 250mah top up after this time. I have eneloops too and they seem to lose about the same, maybe a bit less but not significantly.

 

All of the low-self-discharge NiMHs will lose about 5-10% quite quickly. The difference with normal NiMHs is that the rate then slows down. The exact rated discharge rate depends on the generation (newer are better): https://en.wikipedia.org/wiki/Eneloop. Eneloops are also only charged to about 75% in the factory, which corresponds to a point where the self-discharge is very low (if they charged them to 100% it would be a waste as they would be down to 85-90% by the time they were sold).

 

The main explanation for the top-up after a month, though, is probably more that it's the amount that the charger has to put back in just to be able to detect that the battery is full! You'll probably find that if you charge a battery to cut-off and immediately (within a few minutes) try charging it again it will take 100-200mAh or so, and the batteries don't discharge that quickly! NiMHs also have only about 65% coulombic efficiency over the charge/discharge cycle (so you have to put in about 1.6x the amount of charge you actually used, which is why the standard full charge is C/10 for 16 hours).

[Martin 0]

But being a cheapskate I bought Vapextech at one third of the price - they loose about 5% of charge per day!

 

How are you measuring the charge lost, and for how long does this keep happening?

 

As I note in my reply to Trevor, if you're measuring this by how much your charger wants to put back in, this probably has more to do with how quickly the charger can actually detect the end-of-charge condition. I expect you'll find the batteries slightly warm to the touch at the end of charge, which is normal for NiMH - when a cell is fully charged the extra charge flowing produces gas, which is converted back by a catalyst, wasting the extra energy as heat. (This is also why NiMHs don't need balance charging like lithium chemistries do - some cells will charge fully before others, but these will just get a bit warm while the others finish charging.)

 

The only way to really measure the self-discharge rate of nickel batteries is to get a big load of them, charge them all fully, and then fully discharge them one at a time after the various intervals you're interested in (e.g. discharge one immediately as control, one after 24 hours, one after 48 hours, one after a week, etc). If you want to be scrupulously fair, get even more cells and test more than one (each individually) at each measurement time.

[alan 0]

Dave - If you join all your packs together and charge them at night you could probably cover the National Grid's peak during the day! What do you use 4cell SubC for?

The pack I bought is a 5 cell "instant" AAA.

 

Martin - I have to admit that I can't directly compare Vapextech with Eneloop; The Eneloop I use mostly in a camera and I consider them great because they last many times as long as previous Nimh in the same use. I imagine they are good for Tx and Rx batteries. The Vapextech I am "measuring" using the built in internal battery monitor in the Emeter. It returns a % number presumably based on terminal voltage. I just checked now, 69% compared to 75% yesterday and charged about 4days ago. I can't put them on a "proper" charger as they are now soldered in to the device.

 

Maybe I should cycle them a few times like Trevor said?

[Martin 0]

They may well improve over the first few cycles. Is the device designed for rechargeables or normal primary alkaline cells? If the latter then the 75% may just indicate the lower voltage of rechargeables - I have a DAB radio that I use 2x AA Eneloop in, and fully charged cells only give me 2 bars out of 3 for battery due to it expecting fully charged to be 1.5V not 1.2-1.3V.

 

If the batteries are permanently attached to the device without a proper hard power switch (that physically disconnects the electrical path), then this may just indicate general consumption by the device. I have a digital camera with a LiPo battery which is terrible at retaining charge - nothing to do with the battery but just the device permanently draining a little bit on standby.

 

Martin

[Bravedan 106]

I use NIMH for most Rx packs and all Tx packs, I thoroughly dislike LiPo in a Tx.

 

However, the main use is in the 1/12th scale Mardave RC cars raced at the school club which use the Sub "C" 4 cell packs. Currently I maintain 26 cars, plus at least two spare batteries per car, plus more for juniors who have their own cars. (with two more being built now for the club and three member cars just being started)

 

Most of my own engine powered Helis have 4 cell sub "C" pack, too, because of the high current drain needed for top spec powerful digitals, that's another eight!

 

Then there are all the AAs in Tx's, one Tx per car, and many Tx's are still 27/40 so eight per Tx. THEN there's all the club Heli's and Quad Tx's, another 15 Tx's worth!

 

Some Sub "C" packs in use are from 2007, and a few are now just beginning to drop voltage under high load to below the voltage a 2.4Rx is comfortable with when the car is stalled on full throttle as at start, crash, barrier strike, etc. This is a bad feature of 2.4 with only 4 cell feed, as a 27/40Mhz Rx would instantly recover, whereas 2.4 has the finite reboot time. Hold up capacitors do not work as the brushed car motor and simple resistor speed controller drains them.

 

 

 

My own experience of Instants from various sources has not been that good, though for a while a £1 shop in Littlehampton had some (initially four for a £1, later only two) which did retain charge much better than normal.

[alan 0]

Thanks so much for all the useful and interesting comments. I'll cycle the pack several times (it's built in so I have to leave it switched on with backlight on to discharge) then get back to this thread and let you know what happened.

 

Re. Dave's comments about long lasting nicads; I have some D cells from discarded emergency lighting, they must be at least 10 years old but every now and again I charge up a 24v battery made from these to do some testing. They still take a full charge and they still pack a punch. But a bit heavy!

[Martin 0]

Hold up capacitors do not work as the brushed car motor and simple resistor speed controller drains them.

 

How about a capacitor with a diode to prevent discharge other than by the Rx? Since the speed controller is totally independent of the Rx (being mechanically controlled by a servo) I can't see a reason why not.

 

You'll lose an extra 0.7V, but if that's a big problem you're cutting it quite fine anyway! A 200mA small signal diode would probably be sufficient (getting quick switching), relying on the 1A/2A surge rating for the times when the servos are actually operating.

 

I'd guess that the main problem would be the size of capacitor needed, as while you can protect against the motor draining the capacitor you would still have two servos (and a stalled steering servo will also drain things quickly).

 

This is rather off-topic for the original thread though! Sorry!

 

        /sw       diode
    +-/ +--+------|>|-------+-------+
    |      |                |       |
  +-+-+  +-+-+   +---+   cap|(+)  +-+-+
  |(+)|  |   +---+   |     ---    |(+)|
  |bat|  |SC |   |mot|     ---    |Rx |
  |(-)|  |   +---+   |      |     |(-)|
  +-+-+  +-+-+   +---+      |     +-+-+
    |      |                |       |
    +------+----------------+-------+

[alan 0]

Well, I said I'd get back so here I am. After cycling the built in Vapextech pack several times it appears that I 'm getting 10 hours use, without backlight, for about 80% use of battery. That's practical but Hyperion state 18 to 50 hours depending on use of backlight. Not impressed, I will stick with Eneloop in the future.

[alan 0]

 

....And here I am again. The Vapextech 6v "instant" pack mentioned in this thread has now died and will not accept a charge.

 

OOPS my mistake - charging error. It's now charging.