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Author Topic: IS IS OK TO "TOP OFF" BATTERIES ???  (Read 1203 times)

Posts: 65

« on: April 19, 2005, 08:57:33 PM »

I have a Kenwood THG-71a HT and it came with the pb-39 600mah battery. It was barely adaquet as it runs down too fast. I urdered a new pb-39h 1450mah NIMH battery from batteries america. I havent recieved it yet, but have a couple of questions. I have the standard wall charger from kenwood that takes 15 hours to fully charge. I was told that when i get he new battery to give it 24 hours in the wall charger and then jst "top it off" maybe 3 times a week. I was told to disregard the 15 hour charge and just keep "topping off" the battery. Is this the best thing to do?  I was going to ge tthe fast charger, but thought maybe I could save the $65.00 and just keep using the standard slow charger. I was also told that the fast charger tends to shorten the life of the battery some because of the higher power it puts in. SO I figured I could use my new NIMH 1450mah battery as my standard battery, and keep my NICD 600mah as a spare in case I run dead on the NIMH. Only problem is, if I have a real long qso and happen to run BOTH batteries down, I will have to to recahrge each over seperately in different 15 hour sessions, ugh. How ofton should I recharge the nicd 600mah battery if I am just using it as a spare? I assume it will still run down even if not in use. Any advice anyone could give me about these 2 batteries I would appreciate.

Posts: 984

« Reply #1 on: April 19, 2005, 09:50:21 PM »

From what I understand, you cannot "top off" NiCads because of their "memory" factor. But NIhms are different and topping them does not affect the storage of voltage and does not reduce their life. Maybe some one else will affirm or reject that for us.

Posts: 8

« Reply #2 on: April 19, 2005, 09:58:44 PM »

On the NiCAD battery, I strongly recommend running it all the way down, and not 'topping off'.  NiCAD's are notorious for charge memory, and this is the best way to avoid it.  Using it on the handheld is the best way to do this, too.  I would say about every other month as the interval.

On the NiMH, 'topping off' is OK.  I prefer not to with my own batteries, but that is personal preference.  One caution:  NiMH usually requires a different charge rate and charge controller than NiCAD.  Be very sure that using your 'wall wart' is not going to affect the warrantee on that new battery.  Check with the manufacturer and with the reseller on this.

One advantage of the fast charger is that it will tell you when the battery is fully charged.  The wall wart won't.  Another advantage of the better fast chargers is that they have the sensing circuitry and control circuitry to handle both NiCAD and NiMH without problems.  This is one of the reasons that I have one.  Also, I do recommend that when you do charge a battery, be sure you charge it all the way.

If you have a qso where you run down that new NiMH, the repeater control op may be talking to you, as well.  That is going to be one v_e_r_y long qso. hi hi

Posts: 24

« Reply #3 on: April 19, 2005, 10:47:58 PM »

  As others here have already said, it is not a good idea to top off NiCd batteries, but NiMH are different and the topping off does not shorten their charge hold over time as occurs to NiCd's.

   As far as running your batteries in mid QSO,  I always order the AA battery case option for my HTs when I buy them, and keep it with me.  This allows me to always have power available for the HT, even when the AC mains are not available, by simly purchasing and carrying a supply of AA cels.

   Ed   K7AAT

Posts: 73

« Reply #4 on: April 20, 2005, 06:42:15 AM »

The short answer is maybe. Rechargeable batteries will only cycle so many times. How many is related to the abuse they get over thier lifetime. Nicad's do have memory effect that can be corrected to some extent. NiMh are better in this regard. If you get 500 cycles out of a battery pack before it starts to run down quickly after a charge, you are doing ok. Remember though, that a cycle is counted every time you put the battery on charge. If you treat trhe batteries carefully, you might get close to a thousand cycles, but that is rare. The only real advantage to NiMH over nicad is capacity/size. The NiMH will have a larger current capacity than the nicad in the same size pack. Search this site for more info as there are guys here much more knowledgeable than me.
Rob, ve7rwn

Posts: 6252

« Reply #5 on: April 20, 2005, 08:49:05 AM »

Topping off NiMH batteries does no damage to them, but doing the same to NiCd batteries constantly will actually 'kill' the useful life of the battery in time because of the memory factor.

Running the NiCds down and recharging them will get the most life and useage out of them, but topping them off once in a while because you need the battery fully charged for the next day won't hurt them--just don't do it regularly.

As far as chargers, stay away from the fast chargers unless you absolutely have to have a fully charged battery quickly--those chargers force the charge in--just burning up the batteries over time.  If you use a quick charger you can feel how warm the batteries get--that isn't good for any type of sealed cell battery.

Posts: 5439


« Reply #6 on: April 20, 2005, 10:08:54 AM »

NiCd 'memory' was an artifact of this chemistry years ago, but any modern NiCd does not exhibit this condition.  It need no longer be taken into consideration when utilizing this chemistry.  Even when it was an issue it wasn't as nearly pronounced as some have made it out to be, and it was a reversable condition.

The reason they recommend to top off the battery is because the self-discharge rate for this chemistry is pretty high, just sitting idle it is discharging itself.  It doesn't hurt anything to leave it in a discharged state but if you're depending on it to work at full capacity, you'll need to periodically charge it to replenish what has been lost to self discharge.

In years past, 'rapid' chargers detected when the batteries were full by sensing a rise in temperature of the cell.  This often resulted in batteries that vented, which shortened their life, sometimes dramatically.  It is the heat, not the charge current, that did the damage.  The rapid chargers today sense when the batteries are full by a dv/dt method which occurs well before the cells heat up, and are very safe for the batteries.  I have had excellent cycle life from batteries I've used with these chargers and wouldn't hesitate to recharge them this way as a normal course of operation.

Cycle life (how many times a battery can be recharged before a reduction in capacity) is dependant on how deeply you discharge the battery.  If you run a battery down completely before recharging it, you will get fewer cycles out of it than if you recharged it after every use no matter how far down you discharged it.  You will get more amp hours through a battery during it's life by recharging it when you're done using it than letting it run out before recharging.  With the 'gentle' rapid chargers of today, there is no reason not to top off batteries as often as required.

Bottom line is that the pack's life will depend on how often, and how deeply you discharge it.  If you have two or more packs, rotate them so each sees about the same number of cycles.  When you're done using the rig, put it on the charger no matter how much you've used it.  And, a backup AA pack is always a good idea to take with you, alkalines have excellent shelf life and it will always be ready in case you need it.

Mark K5LXP
Albuquerque, NM

Posts: 2198

« Reply #7 on: April 20, 2005, 01:48:30 PM »

Two points not specifically brought up:

1.  Using the wall-wart charger, it will take approximately 2.4 times as long to fully charge the NiMH batteries than the NiCDd cells!  (1450 mAH/600 mAH)  Assuming your NiCd pack takes 15 hours to charge as you indicated, it will take about 36 hours to charge the new NiMH pack, NOT just overnight or 24 hours.  (Probably why most people DO use a "rapid charger" for NiMH cells in radios, digital cameras, etc.)

2.  NiMH cells have a much greater "self discharge" rate than NiCd cells.  Many sources recommend a charge every two weeks, whereas NiCd cells will retain a usable charge for a much greater time.

Posts: 65

« Reply #8 on: April 20, 2005, 06:43:26 PM »

Wow, I have recieved SOOOOOO much conflicting info about the battery.....Hmmm, now I am more confused than ever. I guess I will need to get the rapid charger after all. I was hoping to save some money. The rapid charger is $65.00 and the battery cost $55.00........I didnt realize when I got the HT that I would need all these extras.

Posts: 3746

« Reply #9 on: April 20, 2005, 08:16:30 PM »

hi john,

With any new battery, it can take a few charge/discharge cycles before the battery can
give you full service.

Check the manual, the radio may have a power save setting, my th-78 has one and that helps conserve the battery.

Early nicad type cells did have 'memory' problem, the nickel would form splinters and puncture the plastic insulation layers inside the cell.  This would impact cell performance.  Most of the time 'zapping' the cell would burn off the splinter(s) and the cell would work fine until the nickel splinter would form again.

73 james
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