Is better to charge the battery only to 80%?

[cooking monkey]

As a new owner of a electric bike this topic is of interest to me. Knowing the state of charge of my battery seems to be indicated by 5 bars rather than a direct voltage measurement. By what I have read I can stay roughly within the best parameters by unplugging the charger at four bars full and never going into the last bar on the display, on discharge.
Currently my bike display shows 4 bars full of 5, when I pulled the battery off the bike I measured 49.5 volts (fluke) at the battery terminals, this from a bike sitting overnight. My question is, if I take a battery voltage measurement this way and try to map a voltage reading to a bar display do I need to wait a bit after I take it off the charger before measuring the voltage? Do I need some time to let the battery sit either after a discharge, or charge before I can consider my measurement accurate?

 

[amberwolf]

If you have a typical ebike battery, and never ever fully charge it, then it isnt' going to be able to balance itself, as that is done on most batteries at the top of full charge.

What this means is that cells taht are lower than others (because of age, or quality / consistency of cells (different batches, etc), and so on) will continue to get lower, and higher cells will continue to get higher. This means that the pack will get lower and lower in capacity, because the low cells will end up with less and less charge in them.

The deeper the discharge is (the more capacity you use out of it each time), the faster this imbalance will worsen.


So if you have a pack that has an imbalance problem, you may need to fully charge it periodically to fix the imbalance. If the imbalance is bad enough, the rebalancing portion at end of full charge can take hours, or even days or more. (this usually only occurs if the pack is greatly different in capacity than it used to be).

Not every pack has a problem soon, some take a long time to get to that point...but they all get here eventually.

 

[Bg-electric-cycle]

I ran on a standard BMS, and changed to 80% charge voltage. Wasn’t long before I were caught out two cells went badly out of balance and the display went from five bars to none.

Wasn’t a bad situation finding a very helpful person in a nearby village was able to store my bike. Great conversation starter.

I ride 10s 13p and the BMS, wasn’t suitable large Ah pack. I now charge with active balancers, the smart BMS, balance feature switched off and via wifi I can keep an eye on the battery pack. When a long journey occurs I charge several times a day to 80%.

Also added an extra halve a volt -charge to 41v- via 2amp charger to compensate for the cold of winter and very impressed with the performance of LG MH1s, given their only protection are few plastic bags. I postulate the slow 2amp charge helps hugely rather than the 8.14amp charger. 6000km+ since June 2018.

 

[dogman dan]

What most likely happened to BG, is that the bms runs on the first two cells. So 24-7, just a tiny bit, its discharging two of your 13 cells more than the others.

If you don't charge to 100%, the bms never fully charges those two cells, and eventually they are not charging to 80%, but 50, then 40, and so on.

So here is the deal, once in a while, charge it full. Do it when the battery is not heated up by a deep discharge, and then ride it down some asap. one hour at 100% is not going to age the shit out of your battery. I define a hot battery as HOT, not body temp, or ambient temp if its a hot summer day.

But running it more and more unbalanced by never charging more than 80%, will kill those first two cells.

As a battery seller, 99% of your customers are not the ES type guy. Best bet for them is just tell em charge it full, when you do charge it. No problem if you ride 4 times before you charge because the ride is super short. Just charge to full, and perhaps repeat it to really get it to balance, at the very least monthly, perhaps weekly. Same schedule all winter too, charge it full monthly. Better than finding two cells dead in the spring.

Definitely charge to full always, if you need that capacity to make it. If you normally use 75%, on some day the weather will cause you to need 105%. On that day, you will still run out before home, but you will be a ton closer when the bms shuts it down.


When people design a system to charge to 80%, part of that design is not a cheap ass bms like you have. With the bms/bulk charger system you have, you still have to charge to 100% to balance the pack. But if you commit to manual pack management, being a human bms, then you can easily avoid the bms running down a few of the cells. And,,, you can balance close enough at 90% charge. At 80%, still a bit hard to do it by voltage measurement. But you can balance to only 90% as little as 4 times a year if you are not whacking the cells out of balance every ride. ( with high discharge rates and discharging deep)

 

[dogman dan]

For cooking monkey, yes, wait a bit for the battery to stabilize after your take it off the charger. half an hour should be plenty.
Two reasons it will drop voltage some.

1 as your battery ages, full is a lower and lower voltage. Brand new perfect full cell is 4.2v, and will stay there. But,,,, betcha you have at most one perfect cell in your pack. Near perfect is still cool, but it may hold only 4.18v. (speaking of the whole string of parallel cells actually) So 13 x .02v is a quarter of a volt down from your full voltage of 54.6v. ( for 13s) So don't panic over a volt lost almost as soon as you unplug. One volt lost may just be your cells quality.

2, your battery may not be full and balanced, when you take it off the charger. So the over full cells will start discharging, which is what the bms is supposed to do. But the charge stopped a bit early because of the over full cells. So you might see that half an hour later, you lost not 1v but 2 or 3. On a good new pack, it should be possible to ride the bike around the block, then put it back on charge, and get back to that only one volt down situation. Or not, refering back to reason 1. As it gets older, full will drop to 4.15v, or less, depending on how good the cells were in the first place, how well the bms worked for a few years, how you rode, etc.

 

[999zip999]

Do you drive your battery out of balance ? By using it over it rated limited or by draining to cutoff as the rule. Do you have quality cells rated for your controller and motor. Are you flogging you battery ?? Driving it out of balance.

 

[DrkAngel]

Yikes ... Getting "gladder" about building my own batteries!
Using care to build using cells free of self discharge and sorting banks to be of of equal IR and capacity has resulted in builds that last for hundreds of cycles with no noticeable bank voltage divergence ... without using any type of BMS!

Though, I admit, most consider my builds to be oversized and undertasked.
Typical eZip battery builds are 25.9V 26-32Ah, (33.3V 32Ah Maxcell Build) sufficient for hours of casual cruising or a week of average commuting.
With most being charged to 4.05-4.10V and seldom discharged below ~3.8V (using Li-Co cells that are "empty" at 3.7V )
Multiple builds have lasted for several years and thousands and thousands of miles each.
2014 26Ah laptop LiPo build still in good condition with nice bank balance at full and "empty". (cold weather IR has increased but runs great during hot Summer months)

PS I do use BMS (more a PCB (Protection Circuit Board) than a BMS (Battery Management System)) for certain builds, most notable in 2s (6V) and 12V (3s) lantern upgrades, just to keep from accidentally running batteries to death. (6v florescent seem to run reliably at 2s ≤8.4V)