Charging pack to 41V experiment
- Thread starter WheezyRider
- Start date
Yep, will do. Not had a chance to do much with the pack so far since getting it back. Hopefully next week.
This thread, post #63 maybe;
www.pedelecs.co.uk
Batteries Charging Routine
I will remind you, but remember, I am interested to know what the charger consumes when the voltage at the output is above 41V. My SANS charger does not let me do that above 41.3V I will use a lab power supply to measure current and voltage for charging and battery tester for discharge.
www.pedelecs.co.uk
Ok, but I'll probably use a higher current than 0.5A, otherwise it'll take too long.This thread, post #63 maybe;
Batteries Charging Routine
I will remind you, but remember, I am interested to know what the charger consumes when the voltage at the output is above 41V. My SANS charger does not let me do that above 41.3V I will use a lab power supply to measure current and voltage for charging and battery tester for discharge.
Yes, but it is a check from 4.2V to 4.1V per cell, so for accuracy, done at 0.5 A.
I would do a capacity check from 41V to 35V at say a couple of amps.
I think there is strong interest in charging at 41v in order to prolong life of the battery and reduce fire risk during charging. However, the cost needs to be quantified, how much loss in capacity? I can get 41v chargers made.
In my experiments around 7 or 8% and most of that is between 41 V and 41.5 V. Above 41.5 V the added energy becomes very small and then when the balancing trips in at around 41.8 V, it becomes hard to tell how much energy is going into charging and what is being consumed by bleed off in the balancing process. It might be better doing such capacity experiments on a single cell, where there is no balancing.
That graph gives you the capacity loss when drained from 4.2V to 4.1V per cell. Which does not tell you what the capacity would be when charged to 4.1V and drained from there.
I used a practical drain load of 2A on my 42V 5Ahr battery as that was the current used when on a typical ride, assist level 2 at 14mph or so.
I noticed that point in your experiment but you have not yet repeated the same experiment with Sturmey 's diode idea.
Not yet, waiting for a 1A charger to arrive.
I want to see if the very extended (150 minute) CV period with the diode used is replicated when the charge current is reduced to a more sensible 0.2C.
So, the capacity check...
I charged the pack to 41V. I left it to stand for a week, then did a discharge test. I found the heating element I had lying around from a heat gun, with the intention of discharging at around 1.5 to 2A, depending on how the resistance of the element changes as it heats up, compared to it's measured resistance at room temperature.
I set up the battery with a new Watt meter. Open circuit, I was getting 41V although it is probably about 40.9 as the Watt meter has an an error of about 0.1V compared to the calibrated DVM.
Initial current was around 1.6A and I took readings at intervals over a 10 hour period.
After 10 hours, I had to go to bed (it was 1am by this time). The pack was down to 33V or so, but it wasn't exactly falling off a cliff at this point and I still think more could have been taken out of it. So from 41V to 33.38, I got 544 Wh.
Here is the raw data:
I was a bit lazy and didn't label the axes, x axis is in seconds, y axis is volts. current is also plotted, but is lost a bit on this scale. You can take the data and re-plot if interested.
So, it's got me scratching my head a bit. These are supposed to be HG2 cells at 3 Ah each, giving a total of 18Ah (648 Wh). I have always been sceptical about this as they were bought from Hong Kong on eBay for £2 each and I never felt that their capacity was that large and the pack seemed to perform similarly to my 15Ah packs. I assumed they were LG but 2.5 Ah versions which had been re-sleeved.
Anyway, if the pack is at 40.9 V, you can expect to lose approx 10% there (compared to a fully charged pack to 42V), so say 64Wh. I would guess there is still probably another 50 Wh to be had if the pack is driven right down to 25 V, which would give a figure just over 648 Wh. Which would mean zero loss in capacity for cells that have done hundreds of cycles and are several years old. However, if you look at Lygate's review of LG HG2 cells:
At 1 to 2 A discharge rate the capacity achieved is less that 2.8 Ah/cell, so 605 Wh for a 6p pack. Hence these cells of mine seem to be giving better results than brand new cells. This makes me wonder if they are actually LG MH1 cells that have been re-sleeved as HG2 cells.
These are nominally 3.2Ah, but were found to give just over 3 Ah when discharged between 1 and 2 A.
So there you go, discuss...
I charged the pack to 41V. I left it to stand for a week, then did a discharge test. I found the heating element I had lying around from a heat gun, with the intention of discharging at around 1.5 to 2A, depending on how the resistance of the element changes as it heats up, compared to it's measured resistance at room temperature.
I set up the battery with a new Watt meter. Open circuit, I was getting 41V although it is probably about 40.9 as the Watt meter has an an error of about 0.1V compared to the calibrated DVM.
Initial current was around 1.6A and I took readings at intervals over a 10 hour period.
After 10 hours, I had to go to bed (it was 1am by this time). The pack was down to 33V or so, but it wasn't exactly falling off a cliff at this point and I still think more could have been taken out of it. So from 41V to 33.38, I got 544 Wh.
Here is the raw data:
| Battery Capacity Test | |||||
| Time (Sec) | Voltage (V) | Current (A) | Watts (W) | Energy (Wh) | |
0 | 41 | 0 | 0 | 0 | |
1 | 40.9 | 1.6 | 65.4 | 0 | |
100 | 40.8 | 1.59 | 64.8 | 2 | |
220 | 40.74 | 1.59 | 64.7 | 4 | |
300 | 40.71 | 1.58 | 64.3 | 5 | |
1080 | 40.42 | 1.57 | 63.4 | 19 | |
1920 | 40.16 | 1.56 | 62.6 | 34 | |
2700 | 39.92 | 1.55 | 61.8 | 47 | |
3600 | 39.66 | 1.55 | 61 | 63 | |
4680 | 39.36 | 1.54 | 60.6 | 81 | |
8520 | 38.36 | 1.5 | 57.5 | 144 | |
10260 | 37.91 | 1.49 | 56.4 | 172 | |
11820 | 37.54 | 1.47 | 55.1 | 195 | |
14580 | 36.88 | 1.45 | 53.4 | 237 | |
16860 | 36.48 | 1.43 | 52.1 | 270 | |
19680 | 36.16 | 1.42 | 51.3 | 311 | |
21720 | 36 | 1.41 | 50.7 | 333 | |
22560 | 35.91 | 1.41 | 50.6 | 352 | |
25200 | 35.68 | 1.4 | 49.9 | 388 | |
28440 | 35.33 | 1.38 | 48.7 | 434 | |
29280 | 35.21 | 1.38 | 48.5 | 444 | |
29820 | 35.12 | 1.38 | 48.4 | 452 | |
30600 | 35 | 1.38 | 48.2 | 462 | |
31200 | 34.88 | 1.37 | 47.7 | 470 | |
32520 | 34.55 | 1.37 | 47.3 | 488 | |
33300 | 34.29 | 1.39 | 47.6 | 497 | |
33840 | 34.09 | 1.39 | 47.3 | 505 | |
34440 | 34.08 | 1.39 | 47.3 | 513 | |
34980 | 33.98 | 1.38 | 46.8 | 520 | |
36060 | 33.73 | 1.37 | 46.2 | 534 | |
36120 | 33.71 | 1.37 | 46.1 | 535 | |
36240 | 33.66 | 1.37 | 46.1 | 536 | |
36360 | 33.63 | 1.37 | 46 | 538 | |
36480 | 33.57 | 1.37 | 45.9 | 539 | |
36600 | 33.52 | 1.36 | 45.5 | 541 | |
36720 | 33.45 | 1.36 | 45.4 | 542 | |
36840 | 33.38 | 1.36 | 45.3 | 544 |
I was a bit lazy and didn't label the axes, x axis is in seconds, y axis is volts. current is also plotted, but is lost a bit on this scale. You can take the data and re-plot if interested.
So, it's got me scratching my head a bit. These are supposed to be HG2 cells at 3 Ah each, giving a total of 18Ah (648 Wh). I have always been sceptical about this as they were bought from Hong Kong on eBay for £2 each and I never felt that their capacity was that large and the pack seemed to perform similarly to my 15Ah packs. I assumed they were LG but 2.5 Ah versions which had been re-sleeved.
Anyway, if the pack is at 40.9 V, you can expect to lose approx 10% there (compared to a fully charged pack to 42V), so say 64Wh. I would guess there is still probably another 50 Wh to be had if the pack is driven right down to 25 V, which would give a figure just over 648 Wh. Which would mean zero loss in capacity for cells that have done hundreds of cycles and are several years old. However, if you look at Lygate's review of LG HG2 cells:
At 1 to 2 A discharge rate the capacity achieved is less that 2.8 Ah/cell, so 605 Wh for a 6p pack. Hence these cells of mine seem to be giving better results than brand new cells. This makes me wonder if they are actually LG MH1 cells that have been re-sleeved as HG2 cells.
Test of LG 18650 MH1 3200mAh (Cyan)
lygte-info.dk
These are nominally 3.2Ah, but were found to give just over 3 Ah when discharged between 1 and 2 A.
So there you go, discuss...
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Your assumption is incorrect. It's impossible to have 10% of the charge between 40.9v and 42v. I alreay showed you that it's about 3%. Any 18560 discharge chart will show the same. That means that your battery lost 7% not zero %. It's impossible to lose zero % after hundreds of cycles. Not even LiFePO4 can do that.
Here is the actual discharge curve of that cell. Let's be generous and say it crosses the 4.1v line at 90 mah. The total capacity is 3270, so 90/3270 = 2.75%, which is close to what I demonstrated with my battery tester.
www.thunderheartreviews.com
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And that is the real World way of doing the capacity comparision;
Stop charging at 41V, measure the capacity.
Wait a bit
Stop charging at 42V, measure the capacity.
Compare the capacities.
Exactly. You have to let the battery rest to get a true picture. When discharging and you hit 41 V, if you stop and let the battery rest it will go back above 41 V after resting. In my discharge experiment, the next day the voltage of the pack had rebounded from 33.38 to 34.2 V.
In experiments where I have charged from 41 to 41.9 V I have been able to put in about 7 to 8% of the pack's rated capacity. Most of that is between 41 and 41.5 V. Once you get to 41.8 V, balancing starts and then it is hard to judge how much is going into the battery and how much is being bled off by the BMS. Even with fairly rapid charging (~4.7 A for 15 Ah pack) I've found charging efficiency to be around 95%.
If the pack is at 40.9 V, ie less than 41V, you lose another % or two of capacity. So overall approx 10%.
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As I said above, you have to let the battery rest in order to find out how much capacity is between 4.1 V/cell and 4.2 V/cell. I have found it to be typically 7 to 8% and this is in line with what is widely reported online.
There are a lot of issues that have to be considered before it can be claimed how much capacity has been lost for this pack.
1) It is not known if these are genuine LG HG2 cells. They have the correct type of wrapping, but they were cheap and were not obtained from a reputable supplier. My guess was that they were perhaps cheaper LG cells re-wrapped in HG2 sleeves, but they could even be some Chinese cells re-wrapped as HG2 cells. Hence we have no concrete idea what the manufacturer's stated initial capacity should be.
2) This experiment was mainly focussed on testing cell balance rather than cycle life. To get an idea of what has been lost, a capacity test should have been done at the start, but I didn't do this.
3) This is a single experiment with a single Watt meter. Even if working perfectly, it will have an error of a % or two and this has not been considered.
4) Say you are right, and it is 3% between 40.9V and 42V. For a 648 Wh pack that would be about 19 Wh. Add that to what I got out in my discharge experiment (544), you get 553 Wh. There is still capacity left in the pack, the voltage is not falling off a cliff, so I think there is easily another 40 to 50 Wh available if charged right down to 2.5 V/cell. This would give about 600 Wh.
Although LG state these their HG2 cell's capacity to be 3 Ah/cell, Lygate tests even at modest discharge rates have found them to be only approx 2.8Ah/cell:
Which would be about 605 Wh for a 10s 6p pack. This is very close.
So, maybe you are right about the capacity between 41 and 42 V, they are HG2 cells, but they have lost about zero capacity...or they are higher capacity but cheaper cells re-sleeved and sold as HG2s, but because we don't know for sure what they are and I didn't do a capacity test from new, we can't draw a firm conclusion over capacity loss.
Although we can't draw definitive conclusions, I still think it is interesting in that after several years of charging to 41V, and hundreds of cycles, the pack is still in balance and capacity is still excellent, with an estimated total capacity of around 3Ah/cell.
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Can you confirm that with your method of reducing the charging voltage (was it NealH who suggested using a diode inline?), the effective reduction in full charge voltage is from 41.7V to 41V? That would giive about 0.7V difference. Saneagle did not give much details how he charged the pack to 41V so it's difficult to interpret his 3% reduction.