Non-solar battery charging question.

tanders12

I realize that this is a solar/wind power website. This question doesn't have to do with either of those things so I apologize in advance if I'm out of line posting here, but you guys seem to know the most about deep cycle batteries and bank charging out of all the sites I've seen.

I perform wireless experiments involving laptops and radios on mobile push carts. I use 2 12v SLA batteries to power the equipment on each cart. Right now I'm using a $50 home depot car charger to charge the batteries. Before you say anything, yes I now know how bad of an idea this is, which is why I'm looking to upgrade my charger. I will probably go with a Powermax but may get an Iota.

So I need to charge up to 16 batteries in parallel. The problem is that since they are constantly moved around, I can't hook up a nice solid set of AWG4 cables to them. Ideally I would like to leave the batteries on the carts. So the system I've designed is to have a dedicated connection from each battery directly to the charger. These connections would be 4-8 ft of 10-12 AWG cable with anderson powerpole connectors. If my understanding is correct, as long as I hook up enough batteries (ie if I have a 55 amp charger I would want at least 6-8 batts connected) then this should still be a safe way to charge, instead of having all the batteries hooked together.


Is this a bad idea for some reason I'm not seeing?

Thanks

EDIT: I believe this link answers my question (from another post on the forum, thanks):
http://www.smartgauge.co.uk/batt_con.html

However if there are other pitfalls or things I should look out for I would appreciate input.

mike95490

Re: Non-solar battery charging question.

It's a bad idea.

You need a charger on each cart. If you have a couple heavily used carts, and connect them to a charger with a lightly used cart, you have full batteries discharging into low batteries, and the charger is doing something else entirely !

Each cart needs it's own AC charger, and should be plugged in to charge, whenever possible.

niel

Re: Non-solar battery charging question.

by sla do you mean agm or gel? agm batteries can't take lots of charge voltage and many of the off the shelf cheapo chargers are not regulated. worse yet would be doing this to a gel battery as they are picky on voltage and current.

the key to charging many parallel batteries is to keep things even, symmetrical, or identical for all batteries and this is not that easy to do. the different drains on the batteries already screws that up. i also don't believe you will get all of your current to the batteries very well with small gauge wires.

BB.

Re: Non-solar battery charging question.

Roughly, the "ideal" plug in charger would charge at 5% to 25% rate of charge... So if you have 12x 15 AH batteries:

• 12x 15AH * 0.05 = 9 amps minimum
• 12x 15AH * 0.10 = 19 amps nominal
• 12x 15AH * 0.13 = 23 amps typical maximum
• 12x 15AH * 0.05 = 45 amps recommended not to exceed

As always, read the specs for your batteries and charge as they recommend--The above are just basic rules of thumbs we start with to size a system.

Also, running long wires from charger to battery bank can slow down charging... If you have more than ~0.05 to 0.10 volts drop with your cabling (from charger to battery), instead of "fast charging" at 14.5 volts, you would be slower charging at 14.0 volts or less (assuming 0.5 volts or more drop). Not the end of the world, but not great.

Also, with that many batteries and wiring mixed together--You should probably add a fuse to each battery pack to limit the chance of a miss-wire or failed wiring connection/shorted battery cell causing too much current flow.

You put that many batteries together in parallel, and you can get 100's of amps flowing into a dead short).

-Bill

tanders12

Re: Non-solar battery charging question.

Great input guys.

@mike90045:
Things become many times more expensive that way. I have 16 batteries... I'm going to put a separate voltmeter on each battery to make sure they are fairly well matched before charging. Would this not work? How closely matched do they need to be? If I were to use a separate charger for each one, is there anything like a BatteryTender but cheaper available?

@niel:
Unfortunately the batteries were already purchased before I came on to the project, and I've been unable to determine whether they are gel or AGM. These are the ones:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_318852_-1
Do you think 12g wire will be too small?

@BB:
Great point on the dead short. Placing separate inline fuses for each battery where it connects to the charger should take care of that, yes?

BB.

Re: Non-solar battery charging question.

Depends on the length of wire and the current you want to pass through it... You can use a generic voltage drop calculator to estimate the drop... Again, if fast recharging is important, I would shoot for 0.05 to 0.10 volt maximum drop when charging a 12 volt battery... If you are able to live with "slow charging", then you might be OK with a 0.5 volt drop.

Also depends on what voltage your charger outputs...

You could ether fuse each battery (may not be necessary) or just fuse the cables leaving the charging bus. In this case, each battery bank is probably small enough that a single dead short on the battery will not fuse the cabling. However, when you have a dozen batteries plugged into one charging bus--A short on that bus could supply the sum of all the rest of the batteries--hence that would be were to put the fuses (to protect against high current flow).

-Bill

niel

Re: Non-solar battery charging question.

i believe those batteries may be gels as the specs list about 2.275v/c which works out to be about 13.65v tops. if you've been openly charging them with a standard charger they may be ruined.

BB.

Re: Non-solar battery charging question.

In general, the maximum charge rate for GELs is around 5% (C/20)... More than that, you can form gas bubbles between the plates and the gelled electrolyte--Which limits the ability of the battery to perform.

-Bill

tanders12

Re: Non-solar battery charging question.

Hm I think the only thing that's been saving us up to this point is that most of the times we charge them we've done a lot of the batteries together, so only a couple amps a piece. That's still higher than C/20 though...

vtmaps

Re: Non-solar battery charging question.

mike90045 wrote: »

You need a charger on each cart. If you have a couple heavily used carts, and connect them to a charger with a lightly used cart, you have full batteries discharging into low batteries, and the charger is doing something else entirely !

Each cart needs it's own AC charger, and should be plugged in to charge, whenever possible.

tanders12 wrote: »

Things become many times more expensive that way. I have 16 batteries... I'm going to put a separate voltmeter on each battery to make sure they are fairly well matched before charging. Would this not work? How closely matched do they need to be? If I were to use a separate charger for each one, is there anything like a BatteryTender but cheaper available?

Yes, its more expensive to have separate chargers, and it will be even more expensive to ruin your batteries trying to charge them with one charger.

As for measuring the voltage on each battery, that will give you no useful information. The voltage will be the same on each battery (that's the nature of a parallel circuit). If there is a difference in voltages it has little to do with the batteries... it is because the wiring is causing a voltage drop. If you want to monitor what's going on, you need a clamp ammeter on each battery. Then you will notice that the current from the charger (which has 16 choices of direction to flow) is uneven in the 16 batteries. Then what do you do?

Bottom line: Parallel battery banks are inherently unstable. Mathematicians would tell you that a pencil balanced on its point is in equilibrium, but it is an unstable equilibrium. So it is with batteries being charged in parallel.

Even if you had 16 perfectly identical batteries (there is no such thing) with perfect parallel wiring you would have an unstable equilibrium because of thermal instability. If one string is a bit warmer than the others (inevitable) it will draw more charging current than the other strings which will make it warmer which will make it draw more current which will make it warmer which will make it draw more current, etc.

--vtMaps

tanders12

Re: Non-solar battery charging question.

vtmaps wrote: »

Yes, its more expensive to have separate chargers, and it will be even more expensive to ruin your batteries trying to charge them with one charger.

As for measuring the voltage on each battery, that will give you no useful information. The voltage will be the same on each battery (that's the nature of a parallel circuit). If there is a difference in voltages it has little to do with the batteries... it is because the wiring is causing a voltage drop. If you want to monitor what's going on, you need a clamp ammeter on each battery. Then you will notice that the current from the charger (which has 16 choices of direction to flow) is uneven in the 16 batteries. Then what do you do?

Bottom line: Parallel battery banks are inherently unstable. Mathematicians would tell you that a pencil balanced on its point is in equilibrium, but it is an unstable equilibrium. So it is with batteries being charged in parallel.

Even if you had 16 perfectly identical batteries (there is no such thing) with perfect parallel wiring you would have an unstable equilibrium because of thermal instability. If one string is a bit warmer than the others (inevitable) it will draw more charging current than the other strings which will make it warmer which will make it draw more current which will make it warmer which will make it draw more current, etc.

--vtMaps

Well they are only hooked up in parallel when charging, so if i make sure the voltages are close before hooking them together, it will reduce uneven cuurent flow.

tanders12

Re: Non-solar battery charging question.

niel wrote: »

i believe those batteries may be gels as the specs list about 2.275v/c which works out to be about 13.65v tops. if you've been openly charging them with a standard charger they may be ruined.

The specs state that they can handle 10.5A, which is the same as the UBG AGMs sold at solar-electric.com. Unless they are really shady with their specs this leads me to believe they are AGM

niel

Re: Non-solar battery charging question.

tanders12 wrote: »

The specs state that they can handle 10.5A, which is the same as the UBG AGMs sold at solar-electric.com. Unless they are really shady with their specs this leads me to believe they are AGM

just because a battery has a similar ah capacity as another does not mean they are the same. agm batteries can certainly take more than 2.275v/c.

tanders12

Re: Non-solar battery charging question.

niel wrote: »

just because a battery has a similar ah capacity as another does not mean they are the same. agm batteries can certainly take more than 2.275v/c.

I was referring to the charge "rate" of 10.5A, not the capacity, which is 35Ah