I am trying to build a battery pack of Honda IMA sticks. I will be using a total of 16 sticks to make 8 14.4 volt batteries, (2 series per module, then parallel together). My question is, what is the actual result of charging in a mass situation such as this where there are packs connected in parallel? Given, I plan to balance them before connecting. However, I would love to not disconnect my parallel buss bars every time I have to charge. If its a matter of simply reducing cell life, Im not overly concerned with that. I will be using these for burst power only, for about 10 seconds at a time. These arent going in place for an EV motor or bike etc. Ultimately, the current draw will be about 400 amps (15v, 6kw). Likely less real clamped power. Any help is appreciated. I am new to NiMH cells. For what its worth, I have extensive use with Carbon nanotube ultra capacitors, which I have used and charged in series/parallel without ever using a balance circuit, nor a balance charger. Ive destroyed 1 or 2 over the years, but never a reactive issue. And yes, Ive researched this topic extensively. I just havent found any testimony from people who have done it against recommendation to see what the actual results may be. Mind you, I wont have much invested in these used cells anyhow. Just trying to see if I can get them to work for this application. Thanks, Kenneth
Result of charging NiMH in parallel circuits?
- Thread starter Ktraughb
- Start date
dnmun
1 PW
how do you expect to balance them, and are these G1 HCH sticks or G2?
I guess thats kinda my question. I intend to balance them only before I connect them parallel. More or less wondering what are the negative effects they will have if they become unbalanced once sitting nominal in parallel? Particularly when I apply a charger again to a bank of potentially unbalanced cells. They will probably be used about once a month or so, and charged before each use. There is potential for a daily charge, but havent decided if thats something I want to do. **hint** If I do charge it on a daily basis (as I did with my maxwell caps) they will also be in parallel with my vehicles alternator/front battery. Again, I saw zero issues with the capacitors being imperfectly balanced (at least in my application). I do understand that a variation in voltage is detrimental to the cell and the intended outcome. For what its worth, I have heard from some reliable sources that they have seen these sticks self balance very closely within 24 hours when connected in parallel. Any validation on that? And to answer, I am unsure of Generation of the sticks. I do know they are from a 2002 insight so they are likely Gen 1.
major
10 kW
The result is typically a fire. NiMH voltage curve takes a dip near full charge which starts parallel cells feeding each other. Once that starts, even turning off the charger will not stop the runaway.
dnmun
1 PW
also nonsense. it sounds like you do not have familiarity with the techniques we use to balance the IMA sticks.
i recently restored my insight pack. in the end i paralleled groups of sticks so that i could drain them down to about .1-.2V while the sticks were connected in parallel. the trick is to find the individual cells in the stick that has held out at a higher SOC so they have some voltage left and by bringing the entire stick down to the low low voltage many of the cells in the stick are reversed.
so the discharge below 5.4V/stick has to be very slow, 200mA max imo. make a balancing tool out of two voltmeter probes with a 5 ohm resistor across the probes. then when you find the cell in the stick which is still at high voltage and the others are reversed, then you can stick the probes into the cell on each end and drain down that high cell so it too is drained down to .2V or so.
these maximum discharges are how we go about restoring the charge storage capacity to the nimh cells which are limited in capacity by the voltage depression that comes from overcharging the cells on a regular basis.
then recharge to full charge so that all the cells in the stick now have the same amount of charge in each cell. use a small balancing charger and keep the balancing current to less than 1/2 of the value they say is safe to overcharge at. (C/10).
on mine i use 300mA led drivers that can push current up to 90V so that tow of them in series can be used to overcharge my entire pack and allow it to rebalance at the full charge level.
i am currently restoring the IMA pack i took out of my civic hybrid. different sticks but same principle.
you do not get good results unless you can effect the discharge down to very low voltage. i am testing how many cycles, and whether it is required to recharge to full voltage in between the 3 discharge cycles that are recommended. each to lower voltage to mitigate the impact of cell reversals.
i recently restored my insight pack. in the end i paralleled groups of sticks so that i could drain them down to about .1-.2V while the sticks were connected in parallel. the trick is to find the individual cells in the stick that has held out at a higher SOC so they have some voltage left and by bringing the entire stick down to the low low voltage many of the cells in the stick are reversed.
so the discharge below 5.4V/stick has to be very slow, 200mA max imo. make a balancing tool out of two voltmeter probes with a 5 ohm resistor across the probes. then when you find the cell in the stick which is still at high voltage and the others are reversed, then you can stick the probes into the cell on each end and drain down that high cell so it too is drained down to .2V or so.
these maximum discharges are how we go about restoring the charge storage capacity to the nimh cells which are limited in capacity by the voltage depression that comes from overcharging the cells on a regular basis.
then recharge to full charge so that all the cells in the stick now have the same amount of charge in each cell. use a small balancing charger and keep the balancing current to less than 1/2 of the value they say is safe to overcharge at. (C/10).
on mine i use 300mA led drivers that can push current up to 90V so that tow of them in series can be used to overcharge my entire pack and allow it to rebalance at the full charge level.
i am currently restoring the IMA pack i took out of my civic hybrid. different sticks but same principle.
you do not get good results unless you can effect the discharge down to very low voltage. i am testing how many cycles, and whether it is required to recharge to full voltage in between the 3 discharge cycles that are recommended. each to lower voltage to mitigate the impact of cell reversals.
dnmun
1 PW
you can follow the discussions on insightcentral where one guy has posted up a chapter from the book on what is happening during the overcharge resulting in voltage depression.
i am discharging my HCH pack right now. i use the little imaX B6 to control the discharge.
i am discharging my HCH pack right now. i use the little imaX B6 to control the discharge.
major
10 kW
major said:
I've personally seen the aftermath from a friend's DIY lawnmower battery consisting of 2 strings of D cells when he parallel charged and could not stop the runaway. Ended up throwing the smoldering smoking glowing mess into a bucket of water. He was lucky it wasn't worse as it was in his garage and he was about to leave home for the evening when he noticed the smell.
I think there have been several discussions on the topic here on ES. Here is one I found where Jeremy had a " massive fire and explosion". http://endless-sphere.com/forums/viewtopic.php?f=14&t=46546
major said:
2 strings per module is exactly what i was planning. in fact, i planned to parallel up to four pairs. which apparently is a big no-no (at least to charge) ! so strange coming from the world of super capacitors and agm sla batteries where charging parallel//unbalanced is not an issue. Obviously I havent done quite enough research on Nimh cells.
Don't bother with these batteries. This was cutting edge technology 15 years ago, but modern lithium makes it a complete joke today.
Actually.. err.. i just bought an insight with a good battery in it and plan to upgrade the battery to lithium, because ( shady reason ).. how about $800 for this primo pack?
:lol:
Actually.. err.. i just bought an insight with a good battery in it and plan to upgrade the battery to lithium, because ( shady reason ).. how about $800 for this primo pack?
:lol:
