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Hotspot battery getting overcharged

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drummerdimitri:

--- Quote from: NiHaoMike on May 15, 2020, 04:26:56 pm ---The ESR of a much smaller battery could result in the usable capacity being greatly reduced if there's a high discharge current.


--- End quote ---

It's not the capacity that is the issue, it's that the hotspot is not reading the voltage correctly and it seems that every time I take it off the charger and put it back on, it starts charging from 2%.

Will need to investigate and test this further...

Peabody:

--- Quote from: drummerdimitri on May 15, 2020, 04:58:56 pm ---
It's not the capacity that is the issue, it's that the hotspot is not reading the voltage correctly and it seems that every time I take it off the charger and put it back on, it starts charging from 2%.


--- End quote ---

Is this with the diodes in place?  How can the hotspot read the battery voltage at all through two reverse-biased diodes?  Well, maybe I just don't understand what you're doing.

amyk:

--- Quote from: drummerdimitri on May 15, 2020, 03:27:15 pm ---
--- Quote from: NiHaoMike on May 15, 2020, 02:40:33 pm ---
--- Quote from: drummerdimitri on May 15, 2020, 12:54:58 pm ---Will be using two diodes back to back to drop the voltage to around 4.2V but currently only have Schottky diodes that drop the voltage by 0.17V so the battery will only be charged to 4.11V thus reducing the effective capacity.

--- End quote ---
That's close to perfect! If you need more capacity, buy a slightly bigger battery.

--- End quote ---

Theoretically speaking, yes close to perfect but now that I have it installed it's far from perfection!

It is reading 23% charged and wont go above that with the diodes in the circuit and it dies when it drops to 0% so I basically cut my capacity in 4  |O

Not sure why that's the case since the voltage drop when the battery is fully charged (4.2V) is only 0.6 V!

Maybe I'm doing something wrong here. A circuit diagram might be useful in this case  :-//

--- End quote ---
In a newer post you said the battery was ~900mAh, and the original 2930. It's probably  using a Coulomb-counter to measure the state of charge, and disconnecting when it reaches 0% to prevent overdischarging, since 23% is awfully close to 900/2930. The diodes are going to confuse it, however, and that's probably where the rest of the missing percentage went --- they're dissipating the voltage drop as heat.

Also, a 900mAh cell in a circuit designed to charge a 2930 may be charged at over 3x its rated current. That will definitely shorten its life too.

The best solution is to find a 4.3V cell (they do exist), if you can't figure out how to modify the charging circuitry to lower the termination voltage.

drummerdimitri:

--- Quote from: amyk on May 16, 2020, 03:44:46 am ---
--- Quote from: drummerdimitri on May 15, 2020, 03:27:15 pm ---
--- Quote from: NiHaoMike on May 15, 2020, 02:40:33 pm ---
--- Quote from: drummerdimitri on May 15, 2020, 12:54:58 pm ---Will be using two diodes back to back to drop the voltage to around 4.2V but currently only have Schottky diodes that drop the voltage by 0.17V so the battery will only be charged to 4.11V thus reducing the effective capacity.

--- End quote ---
That's close to perfect! If you need more capacity, buy a slightly bigger battery.

--- End quote ---

Theoretically speaking, yes close to perfect but now that I have it installed it's far from perfection!

It is reading 23% charged and wont go above that with the diodes in the circuit and it dies when it drops to 0% so I basically cut my capacity in 4  |O

Not sure why that's the case since the voltage drop when the battery is fully charged (4.2V) is only 0.6 V!

Maybe I'm doing something wrong here. A circuit diagram might be useful in this case  :-//

--- End quote ---
In a newer post you said the battery was ~900mAh, and the original 2930. It's probably  using a Coulomb-counter to measure the state of charge, and disconnecting when it reaches 0% to prevent overdischarging, since 23% is awfully close to 900/2930. The diodes are going to confuse it, however, and that's probably where the rest of the missing percentage went --- they're dissipating the voltage drop as heat.

Also, a 900mAh cell in a circuit designed to charge a 2930 may be charged at over 3x its rated current. That will definitely shorten its life too.

The best solution is to find a 4.3V cell (they do exist), if you can't figure out how to modify the charging circuitry to lower the termination voltage.

--- End quote ---

Actually, it is able to charge to 100 percent however, as soon as I turn it off and back on, it thinks the battery is empty displaying 1 or 2 % and dies in a few minutes even though the battery remains fully charged (4.16 V).

Not sure why that is but surely it is not the right solution.

Maybe I'm connecting the diode up incorrectly so if anyone can draw a simple circuit diagram that would be helpful in troubleshooting this issue.

I'm sure there are 4.3 V batteries but without any proper designation, it would be impossible to differentiate them from the "regular" 4.2 V batteries as those are always marketed as 3.7 V nominal  :palm:

Another solution would be to replace the battery with the largest one that would fit the original plastic housing and let it charge to 4.28 V only when being used on the go.

I used to keep the battery in the hotspot while in use so that battery would remain at 4.28 V for months at a time.

Now I will just insert it when I carry it in my pocket to extend its lifetime and just replace the battery ever year or so for the next couple of years and get a new hotspot then which hopefully wont have this issue.



Peabody:
Looking at this page on the Netgear site:

https://kb.netgear.com/24865/AirCard-Mobile-Hotspot-Battery-Information

I see that the 810S uses the W-7 battery.  The picture of the W-7 shows it's a nominal 3.8V battery.  In fact the earlier batteries on that page all say 3.7V, while the later ones say 3.8V.  That may just be marketing, but it also might mean that the 3.8V batteries charge to 4.3V.  If that's the case, your 4.28V charged battery would be ok.  This link indicates that it the case:

https://electronics.stackexchange.com/questions/244732/why-are-3-8v-lithium-ion-batteries-used-in-mobile-devices-rather-than-3-6v-or-3

However, the answers on that page indicate that charging to 4.3V may still reduce the service life of the battery.  An answer for that would be to find or build a 4.2V charger for your batteries.

In any case, you should make sure your battery is a W-7 which is labeled 3.8V.  If you're buying 3.7V batteries, that would explain the issues you're having.

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