Simple battery question

Assuming lead, then yes, you should be find just using buddy pairs of batteries.

The answer to your question is both simple and complicated.

Simple version:
Adding batteries in parallel is not a problem as long as you follow the following:

a. The batteries must be the same Age.
b. The batteries must be the same Chemical.
c. The batteries must be the same Voltage.

If these rules are followed there should be no problems, and you can continue to use the same charger provided the pack voltage has not changed. i.e. 36-volt charger and 2 parallel 36-volt battery packs. One more item, "parallel buddy connections". This is where each 12-volt battery in the 2 strings are placed in parallel with each other as shown here;

Complicated answer:

a. Do NOT mix battery Ages, Types, Voltages, and it is not even a good idea to mix manufactures, as all batteries are NOT created equal no matter what the Specification Sheet says.

You could add a switch to switch battery packs but then you are working each battery pack harder when in use. I would not use a switch, every contact adds resistance which will lower the output from the battery pack. If you use them in parallel you will draw fewer amps from each pack and they will share the load and power the bike or scooter at a lower current draw from each parallel pack. By drawing less power from each individual pack you will increase both the life of the batteries and the overall range. Believe it or not, I have done this, a parallel pack will travel farther than those same batteries with a switch to choose which pack you use.
I had a bike with 2 packs and a switch . I did it so I could ride away from home until I was almost out of power then I could switch to the other full battery pack and know I had enough power to get back without needing a meter, which I did not have at the time. After I put a Cycle Analyst Meter (DrainBrain) on the bike I knew how many Amp Hours were used and remaining in the battery pack, I removed the switch and found I could go about 1.5 miles further than I had been able to go switching battery packs.

Grandpa Chas S.

The simple answer is yes it can be done in several ways which you have mentioned. An important thing is use good safety precautions when performing the wiring which should be done with the same or heavier gauge wiring. You don't want to hook the battery up backwards, short-circuit the batteries, or cause unnecessary damage to the scooter during the process.

If you want a permanent solution, then you could "buddy pair" (hook each 12 volt batteries in parallel with one other battery before connecting the four battery pairs together to make 48 volts total) See diagram below (original batteries are B1-B4 and additional batteries are B5-B8)

+ B1-B2-B3-B4 -
.....|....|....|....|
...B5.B6. B7. B8.

If you want a removable solution, then you could install a connector like an Anderson powerpole (get one big enough to handle the amps) so you could hook a separate 48V battery string in parallel with the original string.

..B1-B2-B3-B4
./...................\
+.................... -
.\.................../
..B5-B6-B7-B8

The original stock charger should charge all the batteries fine either way but will take more time after long rides so you may want to consider a more powerful charger. Also, realize that acceleration, hill climbing, braking, and handling will likely suffer after adding the additional weight. I would also make sure that the new batteries and the additional wiring are mounted very securely.

(Ignore the periods in the above diagrams, they are just for spacing)

One advantage to the two sets of batteries and a switch, or plug, is that the batteries can now be different age, chemistry, or depending on the bike, even voltage. It also allows you to opt not to even carry the second battery on short trips, or to abuse one set, maybe the old ones, and baby another. Two chargers will charge twice as fast as one. It just adds flexibility to the system in my opinion.

Hi,
I am a new member and have a similar question regarding multiple batteries. I have built a dock boat for using on the lake where I live. The Max HP allowed here is 3 and has to be electric. So I have a 55 # Thrust Minn Kota 12v motor hooked to a 12 v deep cycle battery. I have set a Sunsei 500 PV charger on the terminals to trickle charge the battery at 7.5 watts. The motor works well but pulls down the battery quickly. I want to add a second battery ( same everything ) so I get longer time on the lake.
Questions: Do I wire the batteries in parallel as in your diagrams?
Does the PV charger get attached to the same terminals without a switch? A
diode is part of the charger.
And is there a meter that I can include to see how much battery time is
left so I do not get stranded on the lake?
Any ideas?

>>> Questions: Do I wire the batteries in parallel as in your diagrams?
Yes.

>>> Does the PV charger get attached to the same terminals without a switch?
Yes. As far as the charger is concerned it's one battery with twice as much capacity. Bear in mind that it will take twice as long to recharge - and with high capacity deep cycle batteries and a 7.5W charger it's going to take a LONG time to recharge.

>>> And is there a meter that I can include to see how much battery time is left so I do not get stranded on the lake?
Stranded on the lake isn't so bad is it? (you can say to your spouse "sorry I didn't cut the grass I got stranded on the lake" ;-) ). If you want a cheap (but imperfect) solution you could add a digital 12V meter to one of the two batteries and watch it's voltage and when it gets down to a certain level you'll know you need to turn around. You can get a cheap digital meter that plugs into a car's cigarette lighter socket from Wal-Mart for around $10. This is what I did on my gas to electric mower conversion. If you want a much better solution then probably Pak-Trakr (http://www.paktrakr.com) is the way to go - it will give you a digital bar gauge - much more costly ($150).

Ok that is good advice. ( I'll tow the canoe just in case )Could you post the diagram for the two batteries? I was going to connect + to + and - to -. Is that correct? Thanks for the info.

When you take the voltage reading to see how much juice is left in the batteries you must do it when they are under load. In other words with the throttle at maximum. When you remove the load the voltage will return to a much higher voltage and give you a false reading.

Here is the diagram for both types of wiring configurations. Parallel is + to + and - to -.

Grandpa Chas S.

If you run the batterys one at a time, at least you will know when 1/2 is gone.

neardbay is that all your using to charge the battery? That should keep it topped off, but it would take weeks to recharge it and over that time you will damage in the battery due to undercharging it. I would strongly suggest adding even a small 120vac charger or adding more panels. Typically for solar you want a minimum of 2% solar panels sized to the battery, ideally you want 10%. So if you have a 100 amp hour battery you want at least a 2 amp charger or 2000 ma or a 24w solar panel. Ideally you would want a 10 amp or 120w solar panel with a small charge controller. This is over simplified, but gets closer to what you really need.

With your current setup, assuming a 100 amp battery being dead it would take approximately 30 days in full sun to fully recharge it.