Batteries in Parallel
 

Is it safe to connect two robot batteries in parallel? I am working on an offseason project that requires some serious power, and uninterrupted power (running off one battery until it is dead, switching to both, then removing the dead battery while it runs off the secondary battery)

Standard parallelization of batteries to my understanding is okay, but I'm questioning this unique circumstance since the batteries may have different charges (one may be fully charged at 13 volts and the other nearly dead at 11). Will this cause bad side effects or decrease preformance of either battery?

Re: Batteries in Parallel
 

As you may know, a battery can supply huge amounts of current at a relatively fixed voltage. When you connect two batteries directly together, if there is any difference in the voltage, current will flow from the more fully charged battery to the discharged battery. Much current will flow, wires will heat, and a good likelihood of damaging one or both batteries (think internal impedance of the battery and the heat from the I^2 loss) and probable reduction of battery life due to the shedding of lead from plates of the batteries.

Would your application allow the approx 0.7 V loss due to a diode being put in series with each battery, such that the connection of the charged battery would effectively disconnect the discharged battery? The diodes would have to be selected to handle the load current (with a significant margin I would hope).

Re: Batteries in Parallel
 

I agree, putting a fully charged battery in parallel with a discharged battery will cause significant current to flow. Think that the fully charged battery will try to supply charge current to the depleted one while also delivering current to the load. Schottky diodes are available for battery steering that have a lower forward drop. RV stores carry them and they are meant for much higher current than you might encounter. This would allow you to put a battery on line and take a battery off line without an issue.

Re: Batteries in Parallel
 

You can certainly connect lead acid batteries in parallel. Many RVs, boats, and home photo voltaic systems do exactly this. Precautions must be taken to ensure that they are charged and discharged at the same rate, and that the batteries have similar internal resistances over their cycles and their life spans.

Two ES17-12 batteries connected properly in parallel will give you a stable 12 volt supply.

However, rather than asking what you can do with batteries that you have, determine what you need out of a battery and then use one with those specifications.

The spec sheet for the competition batteries is available at http://www.mkbattery.com/images/ES17-12.pdf

This battery is 12 volts. Would a 7.2v or 24v power source let your project run more efficiently? This battery is sealed lead acid. Is that going to be robust enough to be safe for your application, or is it overkill? The battery can supply different very different amounts of energy at different rates, and depending on how low the voltage can drop in your application. Would two batteries supply as much as you really do need?

You must identify what you need to do before you can evaluate any solution.
~Phil

Re: Batteries in Parallel
 

There is a switch commonly available to allow you to do this: Marine Dual Battery Switch

Re: Batteries in Parallel
 

couldnt you just use a bigger battery?

Re: Batteries in Parallel
 

Thank you all for the input. What I'm trying to do is run a 12v to 120VAC inverter off robot batteries. I want to use two so that you can replace one without losing power.

I think I will go with a pair of diodes. This module with two diodes looks about right for the application:
http://www.st.com/stonline/books/pdf/docs/11644.pdf
(the inverter has two 35A fuses, so I'm assuming it will draw no more than 35A)

Thoughts? Should I give it a larger margin?

EDIT: oops, I missed the .74V forward voltage. I'll find a different one.

Re: Batteries in Parallel
 

Uber,
Perhaps you should let us know what inverter you are planning on using and what kind of load. Are the 35 amp fuses in the input side or the output side? Remember that although we demand some pretty high intermittant currents from these batteries, they are not meant for long term high currents. In the constant load at which you are hinting, the manufacturer specification is much lower than the 17 amp hour. At 17 amps, this battery will fall to 10 volts in about 30 minutes. At 30 amps, that falls in about 10 minutes.

Re: Batteries in Parallel
 

Also, what are you planning to run off the inverter? You say there are 2 35A fuses? Sounds like the inverter is capable of running upto 70 amp or 700 watts at 120 volts. Do you really need something this high power? Sure if you run 100 watts off it it would run longer. I would highly recommend buying one that is smaller wattage if you dont need 700 watts. Are you looking to run power in a bus while on the way to a regional ro play games on a TV? Another thing, if the items you wish to power use an adapter try to find an adapter to goes from 12V to their voltage.

-Mike

Re: Batteries in Parallel
 

Also note Figure 2 - if there is a thermal resistance of 15C/w then at 50C you can handle around 10 amps only. In other words, don't just read the shecs, look at (and understand) the graphs.

You might be better off finding discrete diodes in a larger package. One idea would be to find diodes from an old car alternator, they are usually rated quite high. Also, diodes can be put in parallel to increase their current capacity (to a point)

Don

Re: Batteries in Parallel
 

I think you will want to go with a design that has more margin - I might suggest about twice the expected maximum current. Consider something like http://www.st.com/stonline/products/...stps120l15.pdf possibly? Has lower forward voltage drop at maximum current and somewhat lower thermal resistance.

A significant part of the current rating of the devices has to do with the amount of heat the package can transfer away, given that you have provided adequate heat sinking. I could suggest a number of tutorials for selection of the heat sink (e.g. http://www.wakefield.com/pdf/thermal_tutorial.pdf, among others. Just searched on the internet, I have no association with Wakefield).

The tutorials will help you define what kind of power you need to dissipate, how to calculate thermal resistance of the total path, and give you some information on properties of particular materials and thermal grease, and how the surface area per inch of heat sink affects how much you need, and whether you want to blow air over the fan, etc.

Have fun exploring there.

Hope this helps.

Re: Batteries in Parallel
 

I've had good luck with the marine battery selector switches. We have one on our 20' Boston Whaler that works good, and I decided to put on on our T-Shirt Shooting robot, and I can switch batteries without cutting the power at all. You have the choice of running 1, All, or 2, as well as an off position. Diodes are great and all, but for the current that you are running, I have trouble seeing diodes supplying that current without getting hot. The battery switch stays cool and transfers a lot of current (I think ours is rated at like 120AMPS continuous, 200 < 30 seconds, 300 < 10 seconds or something like that...

Re: Batteries in Parallel
 

My dad did this. He took 6 retired robot batteries, chained them up in parallel, put them into a beer cooler, and hooked up an inverter.

Powers his Laptop in the stands for about 6 hours. :)

Re: Batteries in Parallel
 

1075 has built a powered forklift for moving stuff around... its a power wheelchair, with IFI 2004 controls, and 2 12v deepcycle marine batteries... we can run it for days on end, and it doesnt die... but theres no switching out of dead batteries, and they're charged in parallel too, so they should always have the same charge....

Re: Batteries in Parallel
 

The inverter is rated for 500 watts continuous, and 800 watts surge. I do not expect to be drawing this much continuously- at competitions it will be powering a few computers in the stands and possibly a video camera. But it is nice to be able to power small power tools also, which is why I went with the larger inverter. It is pretty efficient, around 80-90%.

I saw that too looking for diodes, but I'm slightly confused about the rating. Does V(F) mean forward voltage, or forward voltage drop? It isn't much good if it can only handle 0.31v forward.

At the moment I like the idea of a switch, but it would need to be an interesting switch: SPDT, but in the center, instead of momentarily opening both terminals, momentarily closing both terminals. The marine battery switch seems to only allow for the first battery or both, never just the second.

Thoughts? Suggestions? Diodes?