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BATTERY RECHARGING DETAILS NEEDED

Discussion in 'Power Electronics' started by HANKMARS, Oct 26, 2020.

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  1. HANKMARS

    HANKMARS

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    Jul 28, 2019
    I have developed some devices who's power requirements are best met with battery packs. At the moment, it is impractical to consider custom battery packages as a power supply. I am constructing my own packs using AA size batteries. Due to space constraints, I am unable to use manufactured, multi-battery, battery holders. I hard wire my batteries together in groups of either 4 or 6 batteries. After the batteries are wired and long leads are attached, I use electrical tape to wrap the pack and keep the desired shape. This method eliminates the ability to unload single batteries, as you would from a battery holder, and insert the discharged batteries into an off-the-self battery charger.
    I am using Ni-MH (Nickel Metal Hydrite) batteries. The nominal voltage of these Ni-MH cells is considered to be 1.2 volts. The particular ones I am using have a 2800 mAh rating. I have no clue as to a maximum efficiency charging current. The reverse charging voltage can be found quickly by simple means, but the optimum charging current is currently a mystery to me.
    I recently read a snippet that stated that the charging characteristics will mirror the battery's power rating. In example, a completely discharged battery with a 2800 mAh rating will reach full charge in approximately 2.8 hours with a charge current of 1 amp. I am reluctant to jump in feet first, and apply a reverse current of 1 amp continuously for even 30 minutes, without at least some general idea as to what to expect as far as reactions like heat production, changes to internal resistance, maximum tolerances of battery to adverse charging reactions, etc. It is my intent to construct battery chargers for my custom battery packs, or better yet find compatible, off-the shelf, chargers. Is anyone here well acquainted with my needed charging requirements, or perhaps could recommend a publication that contains accurate and complete information on this subject ?
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    No idea what you mean by reverse current.
    Normal charge rate is 10 % of capacity.
    Plenty of charge circuits out tbere for nimh and possibly even some dedicated modules.
    Certainly are dedicated chargers one could use such as is used by r/c people.
    Batteryuniversity is a terrific site for any info battery related.
     
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  3. Ylli

    Ylli

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    Jun 19, 2018
    https://batteryuniversity.com/learn/article/nickel_based_batteries NiMH may not be the best choice for you. Charge at the 0.1c rate, unless you can monitor temperature. NiMH batteries do not like to be overcharged, and there is no easily discernible indication of full charge, so you really need to measure how much energy is removed, and then put about 125% back in.
     
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  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Good advice, especially as in a tightly packed array of cells the inner cells cannot get rid of the thermal energy (aka heat) and will heat up much more than the outer cells which can be cooled by the surrounding air.

    There are several chips available that contain a complete NiMh charge controller, e.g. this one. Thse chips include charge termination by monitoring dV/dv and temperature, too.

    Charge balancing (as know for Lithium rechargeable batteries) in NiMh batteries is done by "simply" overcharging the lower capacity cells. Including the thermal stress resulting from overcharging.

    Batteryuniversity as linked by @Ylli is one of my favorite resources when it comes to batteries.
     
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  5. HANKMARS

    HANKMARS

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    Jul 28, 2019
    I am considering battery capacity to be expressed in AMP HOURS which is a function of current flow vs time. That leaves me a little hazy in finding 10% of an Ah. My batteries are rated 2800 mAh. Shopping in the R/C accessory section is a good idea. My battery pack voltages will be 4.8 V and 7.2 V. Thanks.
     

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  6. HANKMARS

    HANKMARS

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    Jul 28, 2019
    I have also heard that overcharging NiMH batteries is damaging. I have accepted the idea that I will have to piggyback a smart circuit on to the charger circuit. The smart circuit would turn off charging current then take a voltage reading of unit being charged. The closer the unit voltage is to target voltage, the more frequently the voltage test is taken until target voltage is reached, then charging ceases.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Why? Amp hours = Ah. The capacity of your batteries is usually stated in Ah (or mAh). If a battery has a capacity of xxx Ah, charge curent should be 0.1 × xxxA. In your case: 2.800 mAh --> 280 mA charge current.
    It is, mainly due to overheating. This is why a good charger reduces charge current and monitors the cell temperature.
     
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  8. bertus

    bertus Moderator

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    Nov 8, 2019
  9. Bluejets

    Bluejets

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    Oct 5, 2014
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  10. HANKMARS

    HANKMARS

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    Jul 28, 2019
    So in theory, a completely discharged cell requires 10 hours of charging at 280 mA? I think it was stated that an advantage to NiMH batteries is that they charge well with out having first to be fully discharged where as is was an important factor to fully discharge a lithium ion (I think it was lithium ion) battery before applying a charging current. I have some battery operated tools with 18 V, 4 Ah batteries that recharge in 30 minutes. Theoretically they would be charging with an 8 amp current. That is a pretty hefty charge current but I suppose possible.
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    11,268
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    Nov 17, 2011
    More. Charging efficiency is only about 80 % or so. Typically charging time is 12 to 14 hours at 0.1C charge current.
    NiMh as well as Lixx batteries do not have to be fully discharged before charging. I think you remember the so called "memory effect" of NiCd batteries which required to completely discharge the batteries from time to time to prevent loss of capacity.
    A well constructed charger can charge a battery with way more than 0.1 C current if the battery is correctly monitored for voltage and temperature to prevent damage to the battery. Usually charging starts with a high current, then charge current is decreased once the battery reaches a certain charge level. Go to Batteryuniversity to learn more about the charging requirements and methods for different battery chemistries.

    The 0.1 C rule presented above is a fairly save charging regimen if you do not have the possibility to monitor the battery's charge state and terminate charging prematurely when the battery is fully charged (or is in danger of being damaged due to e.g. too high cell temperature)
     
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  12. HANKMARS

    HANKMARS

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    Jul 28, 2019
    Right. I am not sure why electron flow was the theory of choice at my school unless maybe that is the method they saw coming in the future. It is based, I believe, on the idea that the negative side of a supply contains an over abundance, or excess of electrons. In MOS technology it appears that that idea is taken into consideration as they use the terms Vss (source) as the negative side of a supply and Vdd (drain) as the positive side of the supply.
     
  13. HANKMARS

    HANKMARS

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    I did purchase an off the shelf charger which works well. Charger is capable of charging 3.6V thru 9.6V battery packs without any outside settings. Very handy in that manner but rather slow. Two to three hour charge times. Thanks for the suggestion.
     
  14. Martaine2005

    Martaine2005

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    RC battery packs 4.8, 7.2 and 9.6v (others too) generally have a ‘slow’ charger if bought as a set. These are roughly 300 mA - 600mA.
    We have ‘field’ chargers too. These are quick chargers and can be any combination from 1A - upwards.
    Never had a problem charging heat shrunk packs.

    Martin
     
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  15. Audioguru

    Audioguru

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    Sep 24, 2016
    You need a battery charger circuit that will properly limit the charging current and detect a full charge (not a full voltage) then turn off.
    since 3 Ni-MH cells will be near 4.5V at their peak voltage then the charger needs a power supply from 6VDC to 8VDC.
     
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  16. HANKMARS

    HANKMARS

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    Jul 28, 2019
    Cool, thanks. Currently have tolerance windows big enough for lengthy charge times but faster times may prove beneficial.
     
  17. HANKMARS

    HANKMARS

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    Jul 28, 2019
    I'm thinking 4.5V is lowest charging voltage. My NIMH cells are rated 1.2V so I figure 1.3V full voltage which equals 3.9V. That leaves .6V ÷ 3 = .2V. That sounds about right for a reverse voltage breakdown level 1.4V per cell.
     
  18. Audioguru

    Audioguru

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    Sep 24, 2016
    Instead of guessing that 1.3V is the full voltage of a Ni-MH cell, I looked in Energizer Battery Company's website for their Ni-MH Battery Applications Manual. It shows that 1.3V is a 10% charge and discharge, it is almost dead.
     

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  19. HANKMARS

    HANKMARS

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    Jul 28, 2019
    According to the discharge graph,( 750mA draw ), the voltage drops from 1.45 to 1.25 during the first 20% of use, then meanders around 1.25V to 1.18V for the next 60% of time in use, then plummets to 0V in a short time. Probably why my single cell battery is rated at a 1.2V output.
     
  20. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Right. 1.2 V is the voltage you can rely on for the longest part of the discharge cycle.
    This is also the reason why 1.2 V NiCd or NiMh cells work surprisingly well in 1.5 V batter applications: Alkaline batteries despite their 1.5 V rating rather quckly drop to around 1.2 V, too, see e.g. here.
     
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