I know you're "not supposed to" because I guess it's possible for them to charge/discharge at different rates and blow up...or something. My deal is I just need more run time from my hacked roomba robot. I've already done it a few times for testing - I just wired a Dewalt battery pack in parallel with the roomba's internal battery and nothing caught on fire - and runtime was more than doubled. I was even more hesitant to let the roomba dock charge both at the same time but that seemed to work ok too, but I'm still concerned about whatever it is that I don't know. I was considering putting it in, but with a high-wattage but low resistance resistor - like 5 or 10 or 100 ohms. The idea being that juice can trickle back and forth without one battery discharging really fast and then getting a huge flow between them. I'm not sure if that's brilliant or moronic. So I have a these options:
Don't do it because I might burn down my robot. And house.
Put them in parallel and let the roomba dock charge them at the same time - no biggie.
Same as #2, but with a resistor between them. (Suggested values?)
Put them in parallel with no resistor, but charge them separately and then ensure voltages are pretty even when connecting them. (I would do this with a microcontroller and relays)
Same as #4, but again with a resistor.
#2 is obviously most attractive to me - the robot monitors it's battery and docks itself when needed and undocks when charged up. My thoughts are the resistor may help prevent drawing too much and burning up my charging dock (or maybe it limits current on it's own - I don't know), but I'm not sure how that might mess up the charger's program/method of charging. ( I do believe the roomba itself does the charge controlling).
Please educate me on batteries and advise which of these are acceptable and which are horrible ideas? What might you do differently - really...the roomba running two raspberry pis and a slew of sensors and gizmos really sucks the battery down too quickly.
The hazard is proportional to the charge/discharge curve differences which are related to the voltages.
You can disregard discharge differences since it simply means some batteries will draw more current than others. Ignoring charge differences could result in overheating if there was overcharging. If the charging circuit shuts off at the nominal voltage then the hazard is limited to those batteries that WILL NOT charge to that voltage. What does all this mean ? It means you need to check all the batteries one by one and write their voltage on a label you stick on the battery. Remove and discard any that are out of tolerance.
NiMH packs work fine in parallel for discharge and are also o.k. if they are being slow-charged (10-16 hours). The only CRITICAL factor is that they have the same rated voltage.
But fast chargers can have a problem because the effect they rely on to tell when the batteries are full can easily get confused when 2 different packs are filling up at different rates.
Given that, option 4 is probably your best bet. A resistor isn't going to do anything useful but it might create a lot of wasted power and so heat.
Hmmm...I see. Maybe I can pull off #4 without removing the 2nd battery by monitoring current flow and using relays so the dock charges one at a time.. the simpler the better.
Or maybe...the internal charge controller monitors the temperature of the internal battery. Thoughts on the intelligence of just monitoring the temperature of the secondary battery, and disconnecting and alerting me if it gets too hot?
jackrae:
Batteries placed in parallel should have the following characteristics :
a) Be from the same manufacturer
b) Be of the same age
c) Be of the same Ah capacity
d) Be of the same rated voltage
e) Be of the same actual voltage
f) Be of identical past work-load
No. Only (d) is required. Now if you were talking series connection then yes, all of the above applies.
If (e) is not true and you connect two batteries in parallel, current will flow between them until they are equal. So long as they weren't too far apart to begin with, this is not a problem.