Operator's Interface: USB Chicklet + Battery

[whytheheckme]

Originally Posted by Al Skierkiewicz

Jacob, the answer to this would certainly be a big No! Batteries in parallel need special handling in order to work properly. There is no substitute for a fully charged battery, charged by a commercial charger in the pit.

Al, I'm not sure that its a problem to put rechargeable batteries in parallel. Remember doing that electricity demonstration in 2nd grade, where putting 9volt batteries in parallel makes the flashlight last longer?

The only reason you may not want to do this is because if the batteries are at different voltages, they may try to charge each other, which with conventional batteries could be a problem, but with rechargeable batteries should be no issue.

Jacob

[Al Skierkiewicz]

Jacob,
When you put two batteries in parallel, the higher voltage battery will pass current to lower voltage battery. A 9.001 volt battery will attempt to push current to a 9.0001 volt battery. As the higher passes current to the lower, it's terminal voltage falls and the process reverses. As this occurs the batteries end up draining each other until there is no more current to be had (domino effect). The only way to prevent this is to use diodes in series with one of the leads (usually the positive). The diodes prevent current from entering the battery but will allow both to supply current to the load. As batteries increase their current density this becomes an additional problem. Our robot main battery for instance could self destruct if placed in parallel with another battery. NiCad and NiMh batteries suffer this as well. Batteries are OK to wire in series but not in parallel.

[Mike Copioli]

Hi Al, I'm gonna have to disagree with you on that one. I assemble 20 amphour NIMH packs for our DPV (dive propulsion vehicles) I have not had one issue with placing cells in parallel. The packs often sit unused for months, still no issue with self discharge other than the standard self discharge rate on NIMH. I perform minimal cell matching proir to assembly and still have had great results. I do the same with our Lion packs. We have had no instances of self discharge. In fact one technique used in cell matching is to place all of the cells in parallel and let them "Equalize". I have used this approach many times and have not had one single issue. Just my thoughts

See you at great lakes.

[Al Skierkiewicz]

Mike,
What voltage are the cells you are using? Everything I have read says this is bad practice and our engineering discussion at lunch this afternoon centered around this as well. Everyone agreed the batteries will self discharge
We are not attending Great Lakes. We will be at Chicago and Boilermaker while I will Lead Inspect at Minnesota as well. Then on to Atlanta.

[Mike Copioli]

The packs are 24 volt 20 amp hour. The cells are 1.2V 10 amp hour cells. there are 40 cells total in series parallel. The Lion packs are 3x 3.6volt 2600mAh cells in parallel placed in series with three alike banks to equal 11.4v at 7.8 Ahr. We have been doing this for years without any issues. I know paralleling cells is an accepted practice. My truck has two lead acid batteries in parallel without any diode protection. Electric vehicles have several batteries in parallel and series. Our defibrilator packs are series parallel even my Lion laptop pack is series parallel. As long as the cells are of equal amphour rating there is no issue with placing them in parallel. They should be matched prior to assembling them but this is not required so long as the weaker cell is not overdischarged. We use a low voltage cutoff switch to ensure that the NIMH cells do not become over discharged and possibly cause cell reversal. Al Check out this link http://www.batteryuniversity.com/partone-24.htm

[seg9585]

I'm not an electrical engineer, but I would imagine that the higher-voltage battery would supply back-current to the lower-voltage battery, thus charging the lower voltage battery until both are at equilibrium. During this process a bit of charge would be lost due to the battery's internal resistance, but this would be a converging process, not a "domino effect".

Batteries are setup in parallel all the time. I am an RC plane hobbyist and it's done to increase the charge capacity to get a longer flight time.

[Mike Copioli]

Originally Posted by seg9585

I'm not an electrical engineer, but I would imagine that the higher-voltage battery would supply back-current to the lower-voltage battery, thus charging the lower voltage battery until both are at equilibrium. During this process a bit of charge would be lost due to the battery's internal resistance, but this would be a converging process, not a "domino effect".

You are correct, that is what happens. Think of it as two pressure vessels placed in parallel. One that contains 2000# of pressure and one that is empty. If both cylinder have the same liquid volume they will "equalize" thier pressure until each cylinder has the exact same volume. In this case the end pressure would be 1000# You will get some unequality at first due to a temp drop in the supply cylinder and a temp increase in the cylinder being filled but in the end the temps will stabilize and every thing will be in equalibrium. This is an analog to voltage and amp hours. Where the difference in voltage would be the pressure difference and Amp hours would be the volume of the cylinders.

[IndySam]

Originally Posted by Mike Copioli

You are correct, that is what happens. Think of it as two pressure vessels placed in parallel. One that contains 2000# of pressure and one that is empty. If both cylinder have the same liquid volume they will "equalize" thier pressure until each cylinder has the exact same volume. In this case the end pressure would be 1000# You will get some unequality at first due to a temp drop in the supply cylinder and a temp increase in the cylinder being filled but in the end the temps will stabilize and every thing will be in equalibrium. This is an analog to voltage and amp hours. Where the difference in voltage would be the pressure difference and Amp hours would be the volume of the cylinders.

Great explanation, thanks Mike.

[Al Skierkiewicz]

Mike,
I respect Mr. Buchmann and read through the Battery University article you linked to above. It still goes against the training I have received so I am researching a little deeper. I have Mr. Buchmann's book at home so I will go through that. What I find interesting is the difference in belief between older engineers and those much younger as if there has been a disconnect (no pun intended). The fact that so many hold true to connecting batteries in parallel as a bad idea and the fact that so many products exist to make this happen (diode arrays, high power Shottky diodes, etc.) makes this a worthy subject of detailed analysis. It may have more to do with battery chemistry than a blanket statement about all batteries. I have observed self discharge in parallel gelcell batteries and wet lead acid batteries when people in the field (field TV production) tried to extend production time. Sometimes this discharge took place over night but certainly over a few days. All production vehicles with a battery supply use either a high current contactor to switch batteries or a diode array to put batteries in parallel. Of course, batteries are charged separetely.

[Dave Flowerday]

Mike & Al,
Sorry for continuing to drag this thread off-topic, but I have a comment: it seems like I see batteries in parallel all the time too. Two examples that come to mind are the large banks of batteries used in electric forklifts and extremely large UPSes. In both cases I've seen banks of batteries that are clearly wired in parallel with large metal bars connecting the battery terminals together (and obviously no diodes involved). I think in these situations it is common to see sub-groups of batteries in series, with the sub-groups then tied together in parallel. I'll have to doublecheck but I think there is an example of this setup sitting not more than 20 feet outside our playing field area in the Motorola warehouse from an old UPS.

Does this sound familiar to either of you? If I remember this correctly then it certainly seems that there are situations where batteries can be wired in parallel without diodes efficiently.
Dave

[Mike Copioli]

The theories taught do suggest that it is a bad practice. However, it in fact is a common practice that works. I have had to contend with the perils of parallel placement of lead acid batteries in my truck and also in the ambulances at my workplace. The perils being when one battery dies(end of life cycle) it takes the other ones with it. That is why it is recommended that all batteries in a parallel bank be replaced at the same time. I do appreciate both theory and practical application. I also believe that whatever works..works. Which suggests to me that sometimes theory is just that...theory. To answer you query David, I have had the same encounters with UPSs and also with Hi-los. Boats do the same thing. There is a selector switch for 1,2 or both. Good luck at all of your regionals. I am looking forward to continuing this discussion at nationals. After we win of course.

Ha ha.