That reverse curve is explained by the fact that as the stick gets to full charge it begins to dissipate the excess energy as heat which reduces the internal resistance of the stick causing a voltage dip for the constant current input.
a bit more detail.
The -dV at peak SoC... is a result of the chemical reaction in the battery cell changing from one type of reaction to another... in the first chemical reaction you are storing energy... once at capacity... that chemical reaction ends and a different one starts... the 2nd chemical reaction does not store energy so all the energy is converted to heat and pressure ... if you look at the temperature log of SoC vs time during a CC charge ... you see the temperature starts to curve up before the -dV ... this happens because even in 1 individual cell... not all molecules inside that cell reach the same point at the same time... so as more and more % of a cell and more and more % of cells begin to change over from the 1st chemical reaction to the 2nd you get more and more of this reaction... thus under a constant current charge rate ... the curving up temperature vs time slope ... then Peak Voltage ... then the dV ... then the -dV ... then the Constant V.
I thought about an automatic shut off, but its not worth the trouble. The time period over which this occurs is fairly long so you have plenty of opportunity to shut it off if you are paying attention at all....and you should be. And, at 240 ma, there would not be enough heat build up to be a problem even if you left it on for a couple of hours or even days extra, just wasted energy. And, since I monitor the strongest stick to make sure all are full, you are already over charging the weakest stick, maybe by several hours, before the strongest one is full.
Its only a phenomenon you can use but not something that is critical....unless you are charging at high currents.
__________________
Jim Isbell
2000, 5 speed, 250,000 miles
"If you are not living on the edge, well then,
you are just taking up too much space."
ImIan, should we be refering to it at "Peak SoC" since that , in the context of the Insight, seems to refer to the gauge on the dash? Maybe we need two terms. I was a bit confused by your post at first until I realized that you were talking about "True" SoC not "Measured" SoC.
__________________
Jim Isbell
2000, 5 speed, 250,000 miles
"If you are not living on the edge, well then,
you are just taking up too much space."
The cute little power supplies showed up today. They are nicely made. I will be an instructor at the UDHEV 3 day class in Mahopac NY this weekend,so I will not get to the charger until next week.
ImIan, should we be refering to it at "Peak SoC" since that , in the context of the Insight, seems to refer to the gauge on the dash? Maybe we need two terms. I was a bit confused by your post at first until I realized that you were talking about "True" SoC not "Measured" SoC.
I suppose we can use whatever terms we like...
True SoC is the context I was speaking in terms of... which refers to the true condition of the battery itself ... not what we think it is... or even what our instruments measure it to be... which is nice to talk about , but is mostly a theoretical type of discussion, as it is very difficult to ever really 100% know the True SoC real time... but is nice to know in order to base other plans on.
Measured SoC would seem to me to require one to measure the SoC in the battery... like during a discharge cycle ... and would only be able to be determined ... what it was when you started ... after you have completed the discharge cycle... this will differ from True SoC , because a measured SoC might include Ah of capacity that are the result of continuing to pull current amps from the battery after the weakest cell has gone into voltage reversal... measured SoC , is useful for battery diagnostics ... but isn't much good for usage, as it only tells you about your initial State A after you reach state B and the battery is discharged.
Calculated SoC , is where we get into taking what we know from True SoC and from Measured SoC tests , to try and calculate the approximate current SoC... always off to some extent.
Calculated Usable SoC , would also be different ... and useful ... but is still not what the dash SoC display tells us.... the dash SoC display tells us something close to , even related to this ... but the dash SoC more has to do with what the BCM is likely to offer from the battery at that time , and not so much what the actual condition of the battery is at that time.
The problem here is that I have no idea what the changing numbers mean. I was making good contact all along, the voltage just kept rising, and now it's very high for a stick that's been sitting for 6-12 months.
I'll give you the out come of this stick later, but it's in the top row, so I won't touch it for about a week.
It was a stupid problem.
#1 the cell had a very high internal resistance and had quite a charge.
#2 the battery in my VOM was almost dead and it was apparently dropping by the second as I tried to read this stick.
The grid charger/balancer prototype is built and charging my spare Insight pack.
Some photos, and a link to the schematic are here: MIMA Pack Whack and rebalancing the battery - MIMA Honda Insight Modified Integrated Motor Assist
The supplies are working as expected, with an unexpected cool feature, the 48V constant voltage supplies also have a foldback current limit, so they will not exceed 600MA. If the pack is really dead/ or les than 120V, the constant current supply cannot reduce voltage to control current, so the 48V supplies will start to foldback and control current to 600MA until the pack voltage reaches the minimum control voltage for the CC supply ~9v,at which point the 350MA CC supply takes over the control, and the voltage they can rise to the max while the 350ma cc is maintained. Not sure how well the CC supply likes the 600MA, but my test of about 30 seconds did not harm it.
Now I need some people with problem batteries packs to bring them here to balance so we can see how well this works.
I started charging the pack at about noon,at an initial voltage of 152.3V. I turned the charger off at 1AM with 170.7V This pack had been sitting in the shop unused for at least 6 months.This morning the pack voltage settled back to 163.4V. The voltage rate of change during the charging was about .1V per 3 minutes, but I did not record the details. A definite slowing of the voltage rise began to be noticed about 11PM and 169V, and I saw the voltage start to dip by .1V at about 169.7. I will do some subpack to subpack voltage comparisons today. I set up the CC supply as the most negative of the four supplies.
Since the three 48V supplies are closely regulated constant voltage, and the negative of the CC supply is at battery pack negative, I can see the change in pack voltage by monitoring only that supply which will run from 9V to 48V as the pack charges. This means that a simple 10:1 divider would allow direct measurement of the charging voltage slope with a 10 bit AD right in the micro. This would allow the micro to determine the rate of change as the pack charges, and could terminate the charge once the pack got to the max and the rate of change dropped to near zero.
During the test, I ran the pack fan at about 11V, and monitored the temperature rise inlet to outlet, and saw no measurable rise above ambient for the whole 12 hour test. The PTC strip was also measured, and it confirmed that there was no measurable rise in temp.
The charger power supplies rose about 5 degrees above ambient, and the CC supply got to about 95F by the end of the test.
Any suggestions as to further test, and further interpretation of the results?
During the test, I ran the pack fan at about 11V, and monitored the temperature rise inlet to outlet, and saw no measurable rise above ambient for the whole 12 hour test. The PTC strip was also measured, and it confirmed that there was no measurable rise in temp.
The charger power supplies rose about 5 degrees above ambient, and the CC supply got to about 95F by the end of the test.
Any suggestions as to further test, and further interpretation of the results?
Mike, I'd run a test with the fan running at the slow speed voltage - whatever that is (6V?) to see if it works, so that you don't have to have it on high all night. I'm fairly confident that it will be enough because 5V handles three sticks charging and discharging at 10amps with no problem.
Also, it would have been helpful to take V readings of each stick before and after. Many fully functional packs will have large variations between the sticks. They shouldn't, but they do. It's important to see that the charger equalizes them. That's the primary purpose of this device. Do you have another pack to test with?
Additionally, 152V is low, but not as profoundly dead as a couple packs I have here. I have a pack that's sitting at about 130V. If you'd like to send me a set of the chargers, I could try it to measure worst-case conditions. To my knowledge, this pack is not damaged, but has been sitting for 3-4 years.
I can also test a trade-in pack that came from a junkyard and then died after the owner installed it in his car and then parked the car for 6 months. Pure P1447. This one will simulate a real world example.
For safety's sake, I'd rather test / charge the 96V and 48V sub-packs separately, as I can leave the breaker off and go in through the rubber plugs. Would this be a major change? I'll want that configuration anyway later for use as an out-of-car top-up charger.
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