What Is Causing This Voltage Drop?

[6zfshdb]

I have two identical good quality 52V, 20 AH Reention DP9 downtube batteries that I purchased from Grin Tech.

52V 20Ah Downtube Battery

A 1040 Wh downtube battery, good for roughly 105 km of typical riding on a charge, with a 40A continuous BMS. 52V (14s) 20Ah with either Panasonic GA or Samsung 35E cells, individually discharge tested at Grin to ensure capacity. UN38.3 certified for shipping.

ebikes.ca

I charge both to 58.8V before a ride and discharge the first to 20% or so. When I switch to the second battery a couple of hours later, the voltage has dropped from 58.8 to 54.6 even though the switch on the second battery is turned off and stored in a pannier. I thought it was a bad battery at first but both do the same thing.

When both batteries are fully charged and placed on the shelf, neither looses more than a few tenths of a volt over time. The voltage drop only occurs during a ride when the spare battery is carried in a pannier or rear rack bag. The battery in use discharges normally with no sudden voltage loss. It appears that motion or vibration is causing the drop in the unused battery.

BTW, I also have several 52V, 17.5AH rear rack batteries that do not loose voltage when carried in a rear rack bag or pannier. The problem only occurs with the DP9 downtube batteries.

Any ideas?

 

[ElevenAD]

Try starting the ride with the back up pack, see if your primary pack also loses voltage while its in the pannier if you haven't already.

 

[PedalUma]

How about putting the spare inside a plastic bag that is padded to test if that helps?

 

[PCeBiker]

Maybe it's the temperature change as the spare battery gets cold for two hours before it's used?
Try storing the charged battery in the cold after a charge and check the voltage.

I read that a lion battery that is fully charged in the cold can absorb more energy then become overcharged and explode in the summer if it was stored at full charge.
Maybe that has something to do with Grins' battery and charger?

 

[Slaphappygamer]

Both batteries do it under the same condition. The batteries are meant for use in a vertical installation. Could it be traveling with them, horizontally, causes the effect? You have a repeatable condition and are close to finding the issue. Perhaps try to travel with the battery in a backpack, just for testing.

 

[6zfshdb]

Try starting the ride with the back up pack, see if your primary pack also loses voltage while its in the pannier if you haven't already.

I tried that. Both batteries display the same voltage drop.

 

[6zfshdb]

Both batteries do it under the same condition. The batteries are meant for use in a vertical installation. Could it be traveling with them, horizontally, causes the effect? You have a repeatable condition and are close to finding the issue. Perhaps try to travel with the battery in a backpack, just for testing.

I carry the spare in a vertical position inside a pannier.

 

[6zfshdb]

How about putting the spare inside a plastic bag that is padded to test if that helps?

I might try that idea, thanks!

I don't understand why the problem doesn't occur when the battery is in use mounted on the downtube though.

 

[PCeBiker]

I don't understand why the problem doesn't occur when the battery is in use mounted on the downtube though.

Perhaps because it is warm and stays warm because it is discharging?

 

[PCeBiker]

Try carrying the battery for two trips without using it and see if the voltage drops even further. (Maybe carry a third battery)

If you are losing energy because of vibration, then there's something wrong. That could be dangerous.

 

[PCeBiker]

I found this article about temperature and lithium cells.
It doesn't explain why your other batteries don't lose voltage though, unless it has something to do with Grins technology?

StackPath

 

[PedalUma]

I might try that idea, thanks!

I am thinking that there is moisture or some other means allowing for a slow discharge, almost as if the inside of the pannier is semi-conductive. It is not arching, just slowly discharging.

 

[6zfshdb]

Try carrying the battery for two trips without using it and see if the voltage drops even further. (Maybe carry a third battery)

If you are losing energy because of vibration, then there's something wrong. That could be dangerous.

The length of the ride doesn't seem to make any difference. The voltage never drops below 54.6V. Since the problem occurs on both batteries which are less than a year old, I'll send an email to Grin Tech and ask their advice.

The problem is just an annoyance and only becomes an issue on extreme range rides. It's a dual battery bike with a 20AH mounted on the downtube and a 17.5AH on a rear rack. I just use the 20AH downtube battery first to avoid the loss. For trips >100 miles, I carry a third in a pannier. I can minimize the problem by making the third battery a 17.5AH rear rack instead of the second 20AH downtube. It's just a 2.5AH difference which is maybe a 10 mile loss of range.

 

[6zfshdb]

I am thinking that there is moisture or some other means allowing for a slow discharge, almost as if the inside of the pannier is semi-conductive. It is not arching, just slowly discharging.

It must be an internal discharge though since there are no hot exposed contacts when the battery switch is off. The battery itself does not get warm to the touch.

This is just a guess but the 4.5V loss might indicate a single string of cells is flaky.

 

[6zfshdb]

Maybe it's the temperature change as the spare battery gets cold for two hours before it's used?
Try storing the charged battery in the cold after a charge and check the voltage.

I read that a lion battery that is fully charged in the cold can absorb more energy then become overcharged and explode in the summer if it was stored at full charge.
Maybe that has something to do with Grins' battery and charger?

This is a possibility but the problem shows up with very little temperature change.

I usually charge to 58.8V the night before a ride using a Grin Satiator. Yes, the battery is warm after a charge but has cooled to room temp by morning. I always check the voltage before heading out and it's never less than 58.6V.

 

[PCeBiker]

It must be an internal discharge though since there are no hot exposed contacts when the battery switch is off. The battery itself does not get warm to the touch.

This is just a guess but the 4.5V loss might indicate a single string of cells is flaky.

Why wouldn't the the battery being used show the same symptoms?
Unless the flaky string is discharging (shorting) and you don't notice it on your voltmeter other than a faster drop of the first 4.5 volts?

 

[harryS]

DP9 is a typical downtube battery. but big.


Seems coincidental that they drop from 58.6V down to the 54.6V of a fully charged 13S 48V battery. Are you reading the voltage off the ebike display? Have you correlated the voltage with an external meter?

Is 54.6V lower or higher than what your bike was showing on the battery that you were using?

 

[AnnaHu]

Does it decrease steadily or fluctuate up and down? If it decreases steadily, the battery is aging. If it goes up and down suddenly, there is a problem with the power management chip inside.

 

[6zfshdb]

DP9 is a typical downtube battery. but big.

View attachment 165631
Seems coincidental that they drop from 58.6V down to the 54.6V of a fully charged 13S 48V battery. Are you reading the voltage off the ebike display? Have you correlated the voltage with an external meter?

Is 54.6V lower or higher than what your bike was showing on the battery that you were using?

Yes, it does seem like I'm losing one of the 14 cell strings for some reason.

I use 3 different meters. One is built into the charger, the second is on the bike display and I have a third handheld gauge that plugs directly into the battery discharge port. All three read within .2V of each other.

Does it decrease steadily or fluctuate up and down? If it decreases steadily, the battery is aging. If it goes up and down suddenly, there is a problem with the power management chip inside.

A good question. I don't know. I'll have to stop and check the voltage several times during a ride to find out.

I suppose I could rig an external gauge and monitor the voltage as the battery is carried in a pannier.

 

[6zfshdb]

Why wouldn't the the battery being used show the same symptoms?
Unless the flaky string is discharging (shorting) and you don't notice it on your voltmeter other than a faster drop of the first 4.5 volts?

It does seem strange that the battery in use doesn't show this sudden drop.