36 volt battery charges to 41.8 volts.

[Lightning P38]

Hi, I have a 36 volt battery that I have used for two years, from ebikekit.com . About 50 discharge cycles from 100% down to 50% or 60% typically. Today I topped off the battery, but this time it seemed it was taking longer than usual, and the charger was warm to the touch, so I checked the voltage with my voltmeter, and the battery was at 41.8 volts. The charger light was still red, so it was still charging. I was thinking it should reach 42 volts.

This is the first time I checked the voltage this year, because things have been humming along.

Am I seeing a slight reduction in battery capacity?
Is there anything I should be watching?

 

[K PierreR]

I would think that 42 volts is too high. I would say you either have a bad charger and overcharging the battery or you are reading high on your voltmeter.

 

[JES2020]

Hi, I have a 36 volt battery that I have used for two years, from ebikekit.com . About 50 discharge cycles from 100% down to 50% or 60% typically. Today I topped off the battery, but this time it seemed it was taking longer than usual, and the charger was warm to the touch, so I checked the voltage with my voltmeter, and the battery was at 41.8 volts. The charger light was still red, so it was still charging. I was thinking it should reach 42 volts.

This is the first time I checked the voltage this year, because things have been humming along.

Am I seeing a slight reduction in battery capacity?
Is there anything I should be watching?

Have you balanced the charge lately?
If you let your cells get out of balance the volts will go down, plus it stresses the battery pack

 

[JES2020]

I would think that 42 volts is too high. I would say you either have a bad charger and overcharging the battery or you are reading high on your voltmeter.

42 v is fine for a 36v battery at full charge.

 

[rich c]

100% charge on a 36v battery is 42 volts. https://electricbikereview.com/forums/threads/battery-voltages.26398/

 

[harryS]

As cells get older, and this is just from time, not from use, they no longer can hold 4.2 V. They might get there with charger attached, but fall off quickly when it's removed. As time goes on, they won't even get there, and stop taking charge at a lower voltage. The charger senses when total current into the battery goes to zero and shuts off.

But it can't hurt to try a balance charge, on the chance that your battery has a balance BMS.

 

[JES2020]

As cells get older, and this is just from time, not from use, they no longer can hold 4.2 V. They might get there with charger attached, but fall off quickly when it's removed. As time goes on, they won't even get there, and stop taking charge at a lower voltage. The charger senses when total current into the battery goes to zero and shuts off.

But it can't hurt to try a balance charge, on the chance that your battery has a balance BMS.

If your battery doesn't have a balance BMS ...time for a new battery pack !
I have had a battery for over 2 years with over 2k miles on it , After a balance charge, I still get a full 54.8 v !

 

[K PierreR]

You guys are right, for some reason I was thinking it was 4.1 volts per cell at full charge, not 4.2.

 

[Lightning P38]

As cells get older, and this is just from time, not from use, they no longer can hold 4.2 V. They might get there with charger attached, but fall off quickly when it's removed. As time goes on, they won't even get there, and stop taking charge at a lower voltage. The charger senses when total current into the battery goes to zero and shuts off.

But it can't hurt to try a balance charge, on the chance that your battery has a balance BMS.

How do I do a balance charge?

 

[tomjasz]

Sometimes a few long monitored charges of 8-10 hours can bring a pack to the full 42v. But it wouldn’t be unhreard of for a battery to be just fine at 41.8. BTW, how are you measuring voltage? Could your measuring device be off by 0.2v? I wouldn’t expect to have a BMS capable of balancing. I’m amazed by how many packs don’t have that feature.

 

[tomjasz]

ebikekit.com

Holy socks! I just looked at their current batteries. $800 for a 48V 10Ah. Goofy!

 

[Lightning P38]

Sometimes a few long monitored charges of 8-10 hours can bring a pack to the full 42v. But it wouldn’t be unhreard of for a battery to be just fine at 41.8. BTW, how are you measuring voltage? Could your measuring device be off by 0.2v? I wouldn’t expect to have a BMS capable of balancing. I’m amazed by how many packs don’t have that feature.

My new voltmeter doesn’t even measure the tenths…which I can’t find since moving. So I am using an older voltmeter, which happens to measure tenths, otherwise I would not have even seen the .2v difference, as the new voltmeter rounded up to 42 volts.
I am going to assume the battery is just fine…it is about 3 years old, so it is aging a bit. So it goes!

 

[JES2020]

How do I do a balance charge?

All you do is leave the charger on for 10 to 12 hours AFTER the full charge is achieved.
As far as NOT having a BMS that balances, read the below extreme safety issues.

The Importance of Battery Management Systems​

Functional safety is of the highest importance in a BMS. It is critical during charging and discharging operation, to prevent the voltage, current, and temperature of any cell or module under supervisory control from exceeding defined SOA limits. If limits are exceeded for a length of time, not only is a potentially expensive battery pack compromised, but dangerous thermal runaway conditions could ensue. Moreover, lower voltage threshold limits are also rigorously monitored for the protection of the lithium-ion cells and functional safety. If the Li-ion battery stays in this low-voltage state, copper dendrites could eventually grow on the anode, which can result in elevated self-discharge rates and raise possible safety concerns. The high energy density of lithium-ion powered systems comes at a price that leaves little room for battery management error. Thanks to BMSs, and lithium-ion improvements, this is one of the most successful and safe battery chemistries available today.

 

[Lightning P38]

Holy socks! I just looked at their current batteries. $800 for a 48V 10Ah. Goofy!

Is that a good price? I paid about $600, for the 36 volt 10Ah battery in my kit, when I priced out the components for my whole kit from them about3 years ago.

 

[Lightning P38]

All you do is leave the charger on for 10 to 12 hours AFTER the full charge is achieved.
As far as NOT having a BMS that balances, read the below extreme safety issues.

The Importance of Battery Management Systems​

Functional safety is of the highest importance in a BMS. It is critical during charging and discharging operation, to prevent the voltage, current, and temperature of any cell or module under supervisory control from exceeding defined SOA limits. If limits are exceeded for a length of time, not only is a potentially expensive battery pack compromised, but dangerous thermal runaway conditions could ensue. Moreover, lower voltage threshold limits are also rigorously monitored for the protection of the lithium-ion cells and functional safety. If the Li-ion battery stays in this low-voltage state, copper dendrites could eventually grow on the anode, which can result in elevated self-discharge rates and raise possible safety concerns. The high energy density of lithium-ion powered systems comes at a price that leaves little room for battery management error. Thanks to BMSs, and lithium-ion improvements, this is one of the most successful and safe battery chemistries available today.

I accidentally left my charger on overnight, and this morning I have 42 volts instead of 41.8 v that I had from the prior charge!

 

[tomjasz]

left my charger on overnight

Exactly what every eBike seller suggests never be done. JES2020 suggested long charges. But they should be monitored.

 

[tomjasz]

Is that a good price? I paid about $600, for the 36 volt 10Ah battery in my kit, when I priced out the components for my whole kit from them about3 years ago.

An EM3ev battery is under $500 and 7 more Ah. IF it’s fits on your frame.

 

[Sparky731]

Hi, I have a 36 volt battery that I have used for two years, from ebikekit.com . About 50 discharge cycles from 100% down to 50% or 60% typically. Today I topped off the battery, but this time it seemed it was taking longer than usual, and the charger was warm to the touch, so I checked the voltage with my voltmeter, and the battery was at 41.8 volts. The charger light was still red, so it was still charging. I was thinking it should reach 42 volts.

This is the first time I checked the voltage this year, because things have been humming along.

Am I seeing a slight reduction in battery capacity?
Is there anything I should be watching?

Can’t speak to your specific charger, but any battery charger must have more pressure (voltage) to charge a battery in order to force electrons into the battery. Example: a 12 volt car battery charger usually charges at 14+ volts.

 

[JES2020]

I accidentally left my charger on overnight, and this morning I have 42 volts instead of 41.8 v that I had from the prior charge!

You're welcome !
there is however, more to the story , if you're interested.

 

[Sparky731]

Have you balanced the charge lately?
If you let your cells get out of balance the volts will go down, plus it stresses the battery pack

You are on point re balancing. Your post prompted me toward more research.

You know this, but I found clarity in why the last 10%-20% of charging time takes longer. From what I can ascertain, this is when balancing takes place as the charging management system parses smaller and smaller armies of electrons into nearly full cells.

This is similar to a computer’s disk management in that it has to scour the disk for available places to store bytes. Also to similar logic used by airlines to rebook passengers onto nearly full flights.

In each case, the larger the array (# of paths to the cells), the faster it will go.

Here is a fun test of a full vs. empty battery. Please do this only with AA batteries: hold, and then drop the battery on-end onto a hard surface from a few inches. The empty battery will bounce. The full will not — evident of the weight of electrons!