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Thread: Battery Replacement for RTC

  1. #21
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    It probably trickle charges the old SLA, which the NiMH cells probably won't like. Most of the better NiMH "trickle charge" circuits are really "maintenance charge" circuits and send pulses of charge current, not a continuous low-level charge.

  2. #22

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    Quote Originally Posted by glitch View Post
    It probably trickle charges the old SLA, which the NiMH cells probably won't like. Most of the better NiMH "trickle charge" circuits are really "maintenance charge" circuits and send pulses of charge current, not a continuous low-level charge.
    True but a while back I had seen this:

    ◦You've probably heard that NiMH batteries cannot be charged in a NiCd charger, and this is indeed true for high-speed chargers. The reason is that high-speed battery chargers must be able to detect the small voltage drop that occurs when a battery reaches its full charge capacity so that it can stop charging (overcharging damages the battery). Older high-speed NiCd chargers usually are only able to detect the relative large voltage drop occurring when NiCd batteries reach a full charge, but most cannot detect the much smaller drop of a NiMH battery -- so the charging continues and the NiMH battery is overcharged (damaged). Note that newer fast chargers designed for NiMH batteries also work fine with NiCd.

    ◦A motherboard uses only a very, very low and steady trickle charge current which is just high enough to maintain the battery in a fully charged state, but not high enough to overcharge. Trickle chargers are very simple designs and do not detect anything. NiCd and NiMH batteries are interchangeable for use with trickle chargers such as used in motherboards.
    Which goes back to Chuck's point on how much current is being supplied. I can check that some time later tonight and report back....
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  3. #23

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    Quote Originally Posted by Chuck(G) View Post
    Not strictly true--what matters more is how much current the charger supplies.

    O.k. I checked the charger and it provides 95 mA at the battery terminals. Based on this:

    The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.
    Given that Eneloop batteries supposedly have a capacity of 2000 mAh this current should be well below overcharging threshold. Also, one has to keep in mid that the batteries are only charging when the computer is ON (i.e. when the big red switch is off no power to the charger and no charging). It is extremely unlikely that your vintage system is running 24/7 and could over charge the battery. At least that's how I see it in my simple non EE mind .
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  4. #24
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    If this were my system, I'd charge at about 1-2 ma tops. When your system is powered on, you're not drawing from the battery--the RTC draws so little when quiescent (powered off), that the very low rate of charge should be more than adequate.

    I'll add that what kills rechargeables is internal heating. When the cell is at full charge, but still drawing charging current, said current goes mostly toward generating heat, which is a killer.
    Last edited by Chuck(G); October 3rd, 2019 at 08:43 AM.

  5. #25

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    Quote Originally Posted by Chuck(G) View Post
    If this were my system, I'd charge at about 1-2 ma tops. When your system is powered on, you're not drawing from the battery--the RTC draws so little when quiescent (powered off), that the very low rate of charge should be more than adequate.

    I'll add that what kills rechargeables is internal heating. When the cell is at full charge, but still drawing charging current, said current goes mostly toward generating heat, which is a killer.
    Really? That low? Would that even do anything in 2-3 hours the system maybe on once a month? I guess I could monitor temps on the batteries for a while and if they feel warm/hot I could always add a resistor to drop the current to the battery from the charging circuit.
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  6. #26
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    Probably not--but recall that, as the battery voltage drops, the charging current increases. 2 ma at full charge isn't unreasonable. Ideally, it should be 0 ma at full charge.

  7. #27

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    Quote Originally Posted by Chuck(G) View Post
    Probably not--but recall that, as the battery voltage drops, the charging current increases. 2 ma at full charge isn't unreasonable. Ideally, it should be 0 ma at full charge.
    True, but my thinking is that most of the time the battery will not be full. I.E. the system will most times be off so the battery will slowly drain out and then when it is turned on the battery will need to be recharged. Of course the usage by RTC and the self discharge rate may be so low that the drain maybe insignificant.
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  8. #28

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    O.k. A small update - the battery holders arrived from China. I did some quick voltage tests (measurements taken after the diodes on the MB):

    3 Alkaline Batteries:
    3Alkaline.jpg

    The voltage on these seemed a bit low. Even though they were supposedly new I am guessing they never had a full 1.5V charge to begin with.

    3 Eneloop NiMH Batteries:
    3NiMH.jpg

    4 Eneloop NiMH Batteries:
    4NiMH.jpg

    The 4 NiMH batteries (fully charges) seemed to deliver a more then adequate voltage. I am not sure if I would trust a 5170 to them but for most clones it should be fine. I think for the actual IBM stuff I would probably source two SLA batteries.

    And the final package:
    PakInstalled.jpg

    The 4x1.5AAA fit in there perfectly. I am currently using the charging circuit. After 2 hours of use (i.e. computer on time) the batteries did not feel warm at all. I will continue to monitor and see what happens.
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

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