Charging NiMH batteries


Last week I sent a message about a N-size battery charger for NiCad batteries. I found some rechargeable N batteries at Radio Shack (as suggested in this forum), but they are NiMH and the don't seem to charge well. Can someone enlighten me on why not?



Hi, Tracy;

I'm surprised with this fact. After reading many posts here, I believe NiMH tend to charge the same way as NiCad's with similar capacity. What I read also is that they have some advantages, liek no memory effect.

Is it possible that because of the fact you found NiMH with larger capacity you need more time to fully recharge them? This is a fact, not a bad thing :).

Let us know more about it, please.

Luiz C. Vieira - Brazil


I wonder how did you charge the battery? I don't think you can charge it inside the 41 as the charging circuit is built in to the battery pack.


Hello, Chan Tran;

I agree with you, there are no specific componets in the HP41 to charge the batteries. Anyway, based on the existing HP41 rechargeable pack circuit, charger output goes straight to batteries contacts. I do not remember if the fullnuts' DC input (the well know golden ball contacts) also go straight to the batteries contacts or if they have some sort of protection, like a series diode or something else. In any case, if the 82120A charging circuit is externally replicated and somehow connected to the calculator's (+) and (-) poles, the only problem I see is the possibility of bad contact between batteries poles and the calculator's poles.

Just wondering about it.

Luzi C. Vieira - Brazil


I have a N-size battery charger (HP PN 92266a) which I used to recharge the batteries. I left the new batteries in the charger overnite and still after about 10 minutes in the 41C the BAT indicator came on.

Perhaps the charger is defective?

Thanks for the support!


Hi, tracy;

do you have the means to measure voltage on each battery separately? Each one should measure from 1.25 to 1.3 Vcc right after being fully charged.

I don't know how does the HP PN 92266a charge the batteries, maybe it charges them as a single 5Vcc unit. If it works this way and if one of the batteries is bad, none of the others will be charged accordingly. On the other hand, if it charges them as four 1.25 units (what is better but unlikely), then each good unit will be charged and if there are bad units, they will not.

In any case, all four batteries are used in series when they are in the HP41, and if one of them is bad, BATT annunciator will appear as the set goes close to 4.5 Vcc.

Let us know if you measure the batteries and what you found.

Best regards.

Luiz C. Vieira - Brazil

Edited: 16 June 2003, 2:24 p.m.


Yes, I think your charger is bad (or the contacts need cleaning). I think the original "N" nicads were 100 maH. Your NiMH cells should be 2-3 times this capacity and a full charge would thus be 2-3 times as long... probably 24 hours should do it (assuming your charger is a standard trickle charger that charges the nicad cells in 10 hours).



Do you have an ammeter? If so, try to measure the current while the batteries are charging. (You will have to insert the ammeter in series with the batteries somehow.)

Just as a guess (depends on the intended charging rate), you should probably have somewhere between 10 and 100 milliamps of current running through the system. (Somebody else may have a better value for the charging current - but if the charger is bad, or one of the batteries is bad, the current is probably much less than this.)

As Luiz indicated, the batteries may be in series or parallel. If the charger operates in parallel, you need to check the current in each branch.


Using the suggestions to my last post, I found that one of the Radio Shack rechargeable batteries is BAD (surprised?) which Luiz suggested might be the problem. I do not have an ammeter handy.

Another question, the MiMH batteries are listed as 1.2V, but when charged and tested they read 1.5V -- is charging the 1.2V battery to 1.5V safe for the batteries or more importantly the 41CV?


Hi, Tracy;

It's actualy good news. In fact, using an ammeter is the best way to measure, because sometimes measuring voltage in a battery without loading in (connect it to a circuit) may give you unexpected good results. If you can measure current across it in a circuit or its voltage in same circumstances, results are the ones you actualy need.

About 1.2V and 1.5V. Regular disposabble alkaline batteries have a 1.5V working voltage, what means that when NiMH are being charged and reach 1.5V they will be "seen" as regular batteries by the calculator, what means no trouble. And rechargeable batteries must be connected to a voltage source that's higher than their own so they become the "load" seen by the charger circuit. As their chemicals recombine to receive "energy", their voltage raises and their internal resistance alters as well. As they reach full charge, voltage measured at the very moment they are disconnected is higher than their nominal. If you let them "rest" for a while and measure again, a small voltage drop will be observed.

If you need more specific data, I believe it's not hard to find papers from battery suppliers.

Best regards.

Luiz C. Vieira - Brazil


When a Nicad or NiMH battery is freshly charged the voltage is around 1.35-1.45 V per cell. As it discharges, it quickly falls to around 1.2V per cell where it stays until it is about to die. Then the voltage falls rapidly to 1.0V per cell.

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