Battery charging advice

[v8DaveP]

Hi,

Not sure this is in the correct area to ask, but i have BH and Cube ebikes both 2016 recently purchased BH is 48v and Cube is Bosch performance line CX 36v.

When i charge them i turn off and disconnect the charger as soon as the light goes green on the BH and when the Bosch system on the Cube display turns off at full charge.

My question is should i always let them charge for some time after full charge is indicated ? i guess i am asking how the BMS system works on ebikes... i have looked and cannot find this specific question in the forums,

 

[Danidl]

Hi,

Not sure this is in the correct area to ask, but i have BH and Cube ebikes both 2016 recently purchased BH is 48v and Cube is Bosch performance line CX 36v.

When i charge them i turn off and disconnect the charger as soon as the light goes green on the BH and when the Bosch system on the Cube display turns off at full charge.

My question is should i always let them charge for some time after full charge is indicated ? i guess i am asking how the BMS system works on ebikes... i have looked and cannot find this specific question in the forums,

Hi my understanding is that it it will not matter at all. Any BMS will include two elements voltage sensors which sample the voltage across each bank of series connected cells and power switches FETs which ensure that the charging current into each bank helps equalise the voltage. .... And switch off current if the voltage drops below safe levels. More sophisticated BMS units as with the Bosch will have additional functions e.g reporting history of charges , imbalances between cells and probably other functions. The Bosch manual suggests that if the battery is not being used, it should be charged to 60% 3 Bars and checked after 6 months

 

[Deleted member 4366]

There's an easy check that you can do. Measure the battery voltage as soon as you take it off the charger. The closer its voltage is to the measured charger voltage, the more equally balanced it is. If there's more than say 0.3v difference, you could try leaving it on for a couple of hours to see if it comes closer.

Without seeing the actual BMS, this may not apply, but most BMSs work like this:

Imagine a line of buckets that you want to be full and all must have exactly the same level of water in them. They're all filled by identical pipes, which all shut off as soon as any bucket reaches full. Over time, with evaporation and spillage there will be an increasing imbalance.

A good solution is to drill a small hole at exactly the level you want to call full, and then change the rule so that the pipes filling them don't switch off until the first bucket is at a level 1 cm higher than full. Now, after the filling shuts off, one bucket will be exactly 1 cm above full and the others will be lower - maybe up to a few mm or more. Any bucket with a level above the level you called full, where the hole is drilled, will drain down to the level of the hole, so all the buckets that had levels above the hole are now exactly at the level of the hole, with maybe one or more still below because it didn't get filled enough, but if you start the filling again, the process will repeat and the lower ones will catch up until all are at exactly the level of the holes.

Each cell group in your battery is a bucket of charge (10 buckets for 36v and 13 for 48v). For a 36v battery, the charger charges to 42.0v in total which is 4.2v per bucket, and each cell group has a bleed resister (small hole) that opens at about 4.17v, so the highest voltage you could get an hour after charging, when the charge above 4.17v has leaked out, would be about 41.7v. The closer the total voltage is to 42.0v immediately after you take off the charger, the more balanced are the cells.

 

[v8DaveP]

Thanks for your detailed replies, i have just finished charging the BH battery from 58% for 4 hours (fully charged green light on) and it reads on my fluke DVM 54.2V , i have now put it back on charge and will measure in a couple of hours. It has done about 110 miles from new and been charged when it gets to about 40%
This battery has two main power pins and 5 small contacts on the battery.

Does that sound like i am doing the right thing?

I'll do the same with the bosch later on today.

Both bikes are charged with the battery on the bike if that makes any difference?

 

[v8DaveP]

Just checked the bh battery after an additional couple of hours charge and it now reads 54.3v directly after charging, so I guess that's ok? Or should charge it for longer?

 

[Deleted member 4366]

You have to check the charger too with the same meter. A reading on its own doesn't mean much because meters aren't always accurate.

 

[v8DaveP]

The charger reads zero volts at the plug, guess it needs to plugged into charging socket for it work?
My DVM is a calibrated one that I used before I retired so I would think it's pretty good.

 

[IR772]

Fluke are the Haibike of meters!

Charging is generally at a constant current say 1 amp, until the charger reaches maximum voltage per cell of say 4.2 volts, the voltage then is fixed and generally charging carries on at the max voltage until the charging current drops to about 10 % of the constant current figure.

The charger then cuts off, there is no trickle charging.

This is the constant current/constant voltage charging method used in good quality chargers.

So once your charger finishes that is it, so switch off or unplug as nothing else is going to happen.

 

[Deleted member 4366]

...but the charger might not be set to the normal voltage of 54.6v.

 

[Danidl]

The charger reads zero volts at the plug, guess it needs to plugged into charging socket for it work?
My DVM is a calibrated one that I used before I retired so I would think it's pretty good.[/QUOTE
Safety considerations based on the low voltage directives would discourage any possibility of contact with a voltage of 60 v. So it is almost certain that there is protection circuitry allowing the full voltage only when the circuit is made. Does your charger have more than 2 contacts. There are different rules for 60 v , 48V moved up from 42V and lower voltages.

 

[Danidl]

I made a mess of my previous message so here goes . If a power supply could even under fault conditions generate a voltage in excess of 60v then it needs additional protection. These conditions are linked to the much derided IEC and E U regulations but are part of international product and person safety. . In the case of the power supply used this will involve a protective loop which will prevent the 57v being applied to applied to exposed terminals. You will probably need that there are more than 2 terminals . on the output of the power supply brick. The voltage protection restrictions are less stringent for 48v and lower and lower again at 24v.
As an example the Bosch charger outputs 5v as a test voltage on a third contact until it makes contact with the battery pack.
Many of the lower cost lower voltage power supplies just use 2 terminals, possibly in RCA phono format and all the intelligence for safe charging is transferred to the BMS in the battery pack.
.

 

[v8DaveP]

I think you're right ... It has 2 larger contacts and 5 small ones.
It's difficult to find a point I could measure the voltage whilst charging.

I'll have a look at the Bosch battery and charger (36V) a see if i can measure the voltage disconnected.

 

[Danidl]

I think you're right ... It has 2 larger contacts and 5 small ones.
It's difficult to find a point I could measure the voltage whilst charging.

I'll have a look at the Bosch battery and charger (36V) a see if i can measure the voltage disconnected.

I have already looked. Bosch have extra contacts on the battery but there is no voltage present on the power terminals when it is disconnected. Again it is a protective measure and presumably requires a link to be made with the carrier before it switches voltage onto these terminals. You will see that there are more terminals on the battery pack than on the charger .

 

[v8DaveP]

Also the Bosch battery measures no voltage on any pins ! But is showing three bars on the bike which is usual after @25 miles

I did measure 4.89V on the charger but I guess that's meaningless.