Volts and Amps question

[capjbadger]

Funny thing is I'm usually adding my 2cents to question like this, but this one stumped me.

I know that batteries in series are additive by voltage (two 12v, 10AH = one 24v, 10AH unit).

I know that batteries in parallel are additive by amperage (two 12v, 10AH = one 12v, 20AH unit).

But both these cases have the batteries as equal units. What happens when you start mixing unequal voltages/amperages? I'm building some solar panels from cells.

For example, what would the math be for hooking up two power sources of say:
12v, 1amp
Plus
0.5v, 5amps

Series? Parallel?

Badger

[Dork]

For example, what would the math be for hooking up two power sources of say:
12v, 1amp
Plus
0.5v, 5amps
Series? Parallel?

Neither.

If you put them in parallel, the higher voltage will over drive the lower one. If you put them in series, the lower amperage supply won't be able to conduct the 5amps and will either restrict it or burn out.

The cells might be forgiving enough to not burn out under the conditions you list, but you probably won't be getting full use of them. You might need a little more circuitry to get them to play together nicely.

What are you trying to do here? Mixing cells you happen to have laying around, or combining them to reach a certain voltage for charging a battery?

[phil]

People recommend connecting batteries of the same chemistry and similar capacities. I believe the issue for using say a 10Amp-hour battery and a 1.5Amp-hour battery is that the 1.5Ah battery will be discharged quicker, and your 10Ah battery will spend some of its energy trying to recharge the dead battery and to run the proverbial electric motor, while your motor will fail to get the power it needs to handle the load.

Conversely, if you are charging batteries of dissimilar capacities, either the charged battery will trick the charger into thinking both are charged and it won't fully charge the bigger battery; or the smaller battery will get overheated and damaged from boiling off the chemistry; or both.

I'm not sure what the situation is if you need, say, 12VDC and you connect a 6V battery and 4 1.5V batteries in series when they have the same Ah ratings. Looks to me like you'd get 12V, but I don't know if there's a problem in there somewhere. On the other hand, if you're charging the batteries, I'd want to consult the C chart. Divide the capacity of the battery by 10 to get the amperage to feed it. A 10Ah battery has a capacity of 10, so 10/10 = 1. You'd feed it 1 Amp or less to charge it. Higher amperage could overheat the battery. See
http://www.cieux.com/bm/batteryWreck.html
for more information on how to damage a battery. :->

[capjbadger]

Neither.

If you put them in parallel, the higher voltage will over drive the lower one. If you put them in series, the lower amperage supply won't be able to conduct the 5amps and will either restrict it or burn out.

The cells might be forgiving enough to not burn out under the conditions you list, but you probably won't be getting full use of them. You might need a little more circuitry to get them to play together nicely.

What are you trying to do here? Mixing cells you happen to have laying around, or combining them to reach a certain voltage for charging a battery?

Hmm...

I'm in the process of building four solar panels to charge four 12v batteries through 4 seperate charge controllers. I have enough cells to put together 4 panels (14.5v @ 1.31A), but I have some cells left over and was hoping I might be able to add them into the panel in parallel to boost the amperage output without screwing with the voltage.

Hmm.. I just looked at other solar panel chargers and they are at around 13v to 18v. I may just add them in series and be done with it after all.


Thanks for the link Phil. That C/10 rule of thumb is good to know. Now I just need to finish building this damn thing so I can get a real world calc of what the power draw from the batteries will be.


Badger

[EspressoDude]

place 24 of the 0.5 volt cells in series = 12 volts

this will contribute 5 amps to the load


put the 12volt in parallel with the series string

this will contribute 1 amp to the load

[gyre]

Battery connectors usually use diodes to prevent some of the problems when using batteries in parallel.
It's even more complex though.
Internal resistance, chemistry,etc.
The advice I have from Optima is to use the identical batteries from the same LOT number.
And their batteries are mor forgiving than most as the internal resistance is so low.
It is easier with those as you can use their deep discharge model for cranking too with little loss.
There are some pretty expensive controllers that seem to be worthwhile.

There probably is a circuit you can make to do this, but I don't know if it will be worth it.

[mdmf007]

Like Gyre said -

In my experience its best to keep things as closely matched as possible or there is problems.

Many times when one of our rigs goes dead, we will throw a smaller battery in the mix to start it, leave it there too long or forget to replace it with the normal batttery and its toasted.

You can place voltage regulators in the syste,, but that just lowers your efficiency, and costs more than the batteries .

[Somnivore]

Don't mean to sound stupid, but can one of you guys review the connections for parallel vs series (I can't remember which is which). I know I can google this but figured it might be helpful for others here as well. Thanks, Som

[EspressoDude]

batteries stacked in a flashlight are in series

[Somnivore]

Now I really feel stupid, never thought to remember it that way. Was trying to remember it by how you run subwoofers (series vs parallel). Thanks for the quick reply!