Ebike 36v battery reading higher voltage when switched on vs off

[qwertyman900]

I have a 36v 25ah battery, I'm trying to do a balance charge, has been on a few days only getting to 41.4 seems to have stopped but being a larger battery am sure will take a long time to balance, is fairly new , good quality cells, good quality voltage metre, while reading the voltage I noticed the battery was switched on ,so I switched it off , not thinking would be much difference as no real draw, thinking maybe it should on for the bms ? anyway when I switched the battery off it reads 41.7 , when switched on 41.4, why the big difference and should it be switched on or off while charging ?? by switched on I mean the switch on the battery , not connected to the bike, edit , I'm now thinking off for sure , as if the voltage is higher when off then that's the voltage I want , I don't want to charge to 42v when on then switch off and goes over 42, so yeah off , will leave on another day or so see if can get to 42, then I will only charge 80% for most uses or 39.5v with an occasional balance charge, I think people should think of this more , the last 20% of the charge is what I hear "stresses" the battery,so buy a bigger battery than you think you will need and keep it in the sweet spot range, ie only charge to 80% and don't completely discharge and will hopefully last a lot longer, saying that will be prob old "tech" soon anyway lol, anyway still wondering why the difference in V vs on/off ?

 

[Nealh]

My theory is when switched on the bleed resistors are more active and are open hence one is seeing the 0.3v difference vs charging with the battery off.
I always charge with the switch on as it switches on the BMS .

One will have to record & and measure SOC and see what occurs, charge with battery on and then check SOC with battery on & off. Leave battery to sit with BMS on and see if it bleeds cells down or not and vice versa carry out the same with BMS switched off to see if cells self bleed down.

With this type of battery only carrying out an 80% SOC is imv likely to cause more issues ion the long run with imbalance occurring as the BMS is a dumb generic model with no way of knowing what an inbetween balance or SOC % is , it has no way of balancing for 80% . To carry out a 80% charge one really needs to have a smart programmable BMS fittted where one can alter the parameters of balance top voltage and also to monitor individual cells via BT or uart PC connection.

We used to do it with LIPO packs but the chargers used were different and used single cell monitoring with adjaustable balance features. Lipo certainly suffered from to low or high SOC.

The advantages and life cycle gains for an ebike battery is probably small over the battery life cycle. The 80% rule has likely expanded from the storage charge rule of not storing long term at above 80% soc.
Any past experiments with such SOC processes is useually only based on single cell testing and not multiple parallel cells.
If the battery is in regular use personally I wouldn't bother with making things more complecated and possibly introducing future issues that may mean more manual time consuming actions to put thiings right.

 

[saneagle]

I have a 36v 25ah battery, I'm trying to do a balance charge, has been on a few days only getting to 41.4 seems to have stopped but being a larger battery am sure will take a long time to balance, is fairly new , good quality cells, good quality voltage metre, while reading the voltage I noticed the battery was switched on ,so I switched it off , not thinking would be much difference as no real draw, thinking maybe it should on for the bms ? anyway when I switched the battery off it reads 41.7 , when switched on 41.4, why the big difference and should it be switched on or off while charging ?? by switched on I mean the switch on the battery , not connected to the bike, edit , I'm now thinking off for sure , as if the voltage is higher when off then that's the voltage I want , I don't want to charge to 42v when on then switch off and goes over 42, so yeah off , will leave on another day or so see if can get to 42, then I will only charge 80% for most uses or 39.5v with an occasional balance charge, I think people should think of this more , the last 20% of the charge is what I hear "stresses" the battery,so buy a bigger battery than you think you will need and keep it in the sweet spot range, ie only charge to 80% and don't completely discharge and will hopefully last a lot longer, saying that will be prob old "tech" soon anyway lol, anyway still wondering why the difference in V vs on/off ?

Where are you measuring the voltage? You should measure on the charge socket if the battery is assembled. If you've opened it, you should measure the cell-pack.

Obviously, if you're measuring the output terminals and you switch it off, you're going to get a lower voltage. The voltage you see on the display depends on the impedence of your meter - anything from 0v to full cell-pack voltage.

Some batteries need to be switched on to charge. You can see that by whether the charger light is red or green.

You should always switch on the charger so that the jack is live before inserting the jack, otherwise you can do some serious damage when the jack gets welded to the socket by the inrush spark, and you try to pull it out.

Why are you trying to do a balance charge? Every charge should be a balance charge!

 

[Sturmey]

I have a 36v 25ah battery, I'm trying to do a balance charge, has been on a few days only getting to 41.4 seems to have stopped......... should it be switched on or off while charging ?? ..........

To answer your question, I have come across two new different batteries with on/off switches with instructions and the instructions in both cases is that the batteries should be switched on when charging.
Its not unusual for batteries to settle at an end voltage of say 41.6v. The last BMS I fitted quotes a balance voltage of 4.18V +/- 20mV per cell.
The last battery I bought and tested (48V) reads 54.2V fully charged which works out at 4.17V per cell. The charger reads 54.7V at the plug end open circuit. Of course the meters could be out a little as we are talking of differences of less than 1%
So in your case, I would say turn on your battery, charge till the green light come on and I would think everything is probably OK.

 

[Sturmey]

Where are you measuring the voltage? You should measure on the charge socket if the battery is assembled. If you've opened it, you should measure the cell-pack.

Obviously, if you're measuring the output terminals and you switch it off, you're going to get a lower voltage. The voltage you see on the display depends on the impedence of your meter - anything from 0v to full cell-pack voltage.

Some batteries need to be switched on to charge. You can see that by whether the charger light is red or green.

You should always switch on the charger so that the jack is live before inserting the jack, otherwise you can do some serious damage when the jack gets welded to the socket by the inrush spark, and you try to pull it out.

Why are you trying to do a balance charge? Every charge should be a balance charge!

That cheap battery I bought last October seem to behave the same. I think you may have also bought the same. I charged my 48v battery this morning and took the following readings after the charging was complete and the green light was on the charger. The charger is still plugged into the mains and connected to the battery.
With the battery switch in the on position, the battery reads 54.2 volts at the output xt60.
When the battery switch is turned off, the output rises to 54.5.
When the charger is disconnected, the battery voltage drops to millivolts. (Battery turned off)
When the battery is turned back on again, it reads 54.2 volts (charger is disconnected).

I think that the higher voltage reading when the battery is turned off and charger still connected comes from the charger. The battery is as below. It seems a good battery and is in daily use at the moment..

 

[Sparksandbangs]

You should always switch on the charger so that the jack is live before inserting the jack, otherwise you can do some serious damage when the jack gets welded to the socket by the inrush spark, and you try to pull it out.

Surely the opposite is the case. Having a live jack pushed into a socket is more likely to cause an arc?

 

[saneagle]

That cheap battery I bought last October seem to behave the same. I think you may have also bought the same. I charged my 48v battery this morning and took the following readings after the charging was complete and the green light was on the charger. The charger is still plugged into the mains and connected to the battery.
With the battery switch in the on position, the battery reads 54.2 volts at the output xt60.
When the battery switch is turned off, the output rises to 54.5.
When the charger is disconnected, the battery voltage drops to millivolts. (Battery turned off)
When the battery is turned back on again, it reads 54.2 volts (charger is disconnected).

I think that the higher voltage reading when the battery is turned off and charger still connected comes from the charger. The battery is as below. It seems a good battery and is in daily use at the moment..
View attachment 63172

If you have the charger switched on and connected to the battery and the battery is switched off, you're measuring the charger voltage, not the battery voltage.

When you switch on the battery in that situation and it'sfully charged, there will be a slight load on the charger, so the voltage will drop a tiny bit. The slight load comes from the bleed resistors on each cell string (13 for a 48v battery and 10 for 36v). Each one is 100 ohms, so will dissipate 4.2 x 4.2/100w, so 2.3w total.

Without the charger connected the battery voltage will drop to whatever the voltage is that the bleed system is set to, which is typically in the range 4.18v to 4.2v per cell, so a settled fully-charged 48v battery would be somewhere between 54.34v and 54.6v if fully balanced.

 

[Cadence]

On a slightly related topic, I have a dilemma that I'm struggling to understand. When I switch my Yosepower 36v.13A batteries off the reading at the discharge socket is zero - as I would expect.
My newly purchased PSWPower 48v. 15A battery charges to 54.3v., which I consider to be OK.
However, I've just checked the discharge voltage after a short ride (so not fully charged). The voltage reads 52.8v. when switched on and and when I switch off I get a reading of 45.5v. I would have expected to get zero like my other batteries. Despite this, if I switch off the battery while it's on the bike the display does turn off. I'm sure there must be a logical explanation for this but still having voltage at the discharge socket when the battery is turned off is a bit of a surprise to me?

 

[saneagle]

On a slightly related topic, I have a dilemma that I'm struggling to understand. When I switch my Yosepower 36v.13A batteries off the reading at the discharge socket is zero - as I would expect.
My newly purchased PSWPower 48v. 15A battery charges to 54.3v., which I consider to be OK.
However, I've just checked the discharge voltage after a short ride (so not fully charged). The voltage reads 52.8v. when switched on and and when I switch off I get a reading of 45.5v. I would have expected to get zero like my other batteries. Despite this, if I switch off the battery while it's on the bike the display does turn off. I'm sure there must be a logical explanation for this but still having voltage at the discharge socket when the battery is turned off is a bit of a surprise to me?

As I said above, the voltage you see on your meter when the battery is switched off depends more on the impedence of your meter than anything else. As soon as you connect any load, it'll collapse to zero volts.

What happens is that some charge can leak through the MOSFETs, so the full voltage appears on both sides of them. If you had a perfect meter, you'd always see the full voltage on the off side; however, that charge has to go through your meter to give the measurement, and that tiny current is more than what the leakage is, so the voltage collapses. The voltage you measure can't be used for anything. It wouldn't even light an LED.

 

[qwertyman900]

My theory is when switched on the bleed resistors are more active and are open hence one is seeing the 0.3v difference vs charging with the battery off.
I always charge with the switch on as it switches on the BMS .

One will have to record & and measure SOC and see what occurs, charge with battery on and then check SOC with battery on & off. Leave battery to sit with BMS on and see if it bleeds cells down or not and vice versa carry out the same with BMS switched off to see if cells self bleed down.

With this type of battery only carrying out an 80% SOC is imv likely to cause more issues ion the long run with imbalance occurring as the BMS is a dumb generic model with no way of knowing what an inbetween balance or SOC % is , it has no way of balancing for 80% . To carry out a 80% charge one really needs to have a smart programmable BMS fittted where one can alter the parameters of balance top voltage and also to monitor individual cells via BT or uart PC connection.

We used to do it with LIPO packs but the chargers used were different and used single cell monitoring with adjaustable balance features. Lipo certainly suffered from to low or high SOC.

The advantages and life cycle gains for an ebike battery is probably small over the battery life cycle. The 80% rule has likely expanded from the storage charge rule of not storing long term at above 80% soc.
Any past experiments with such SOC processes is useually only based on single cell testing and not multiple parallel cells.
If the battery is in regular use personally I wouldn't bother with making things more complecated and possibly introducing future issues that may mean more manual time consuming actions to put thiings right.

Thanks for the info , the 80% charge throwing out of balance is interesting , thank you , I actually just tried to charge to 39.5 last night but left charging , I thought oh well , at least will be balanced again and have a nice full charge, today is sat at 41.v, wondering whether will stay there and my 80% charging is already starting to show an imbalance , I may just charge fully from now on unless storing, thanks, edit , just checked again has moved to 41.5v, I did switch if off as I took the 41v reading about 30 mins ago , either switching off allowed it to top up , or was a coincidence , thanks for the replies..gies ...................hmmmmmmmmmm 2nd EDIT***, I just checked voltage measured out battery terminals , with charger in , says 41v when switched on and 41.5 v when switched off , which is a different difference than before at 41.4v on and 41.7 off lol , ok so i measured with charger out , reading 0.3 off and 41v on , then with charger in again , measured 41v on and 41v off while holding "prongs" and switching on and off , stayed at 41v , until I removed prongs switched on and off, then back to 41v-41.5 , is not an issue just thought would share this"weird" confusing info to me lol, am sure is a simple answer and has already been answered in this thread with the replies, just thought would share anyway , thanks guys

 

[saneagle]

Thanks for the info , the 80% charge throwing out of balance is interesting , thank you , I actually just tried to charge to 39.5 last night but left charging , I thought oh well , at least will be balanced again and have a nice full charge, today is sat at 41.v, wondering whether will stay there and my 80% charging is already starting to show an imbalance , I may just charge fully from now on unless storing, thanks, edit , just checked again has moved to 41.5v, I did switch if off as I took the 41v reading about 30 mins ago , either switching off allowed it to top up , or was a coincidence , thanks for the replies..gies ...................hmmmmmmmmmm 2nd EDIT***, I just checked voltage measured out battery terminals , with charger in , says 41v when switched on and 41.5 v when switched off , which is a different difference than before at 41.4v on and 41.7 off lol , ok so i measured with charger out , reading 0.3 off and 41v on , then with charger in again , measured 41v on and 41v off while holding "prongs" and switching on and off , stayed at 41v , until I removed prongs switched on and off, then back to 41v-41.5 , is not an issue just thought would share this"weird" confusing info to me lol, am sure is a simple answer and has already been answered in this thread with the replies, just thought would share anyway , thanks guys

Don't do that stupid 80% charging. It doesn't work for ebike batteries. Don't take any notice of what all the stupid people say how well it works for them. You should always charge your battery to full if you can. The only time you should charge to anything else is to charge to 37v if you plan to store your battery for a long period.