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-   -   Can I mix different CCA battery ratings? (http://www.diychatroom.com/f18/can-i-mix-different-cca-battery-ratings-143154/)

Red Squirrel 05-08-2012 07:32 PM

Can I mix different CCA battery ratings?
 
I bought a 12V marine battery and an inverter-charger at Canadian Tire so I can use it as a UPS. Turns out the inverter-charger's switch time is too slow for use with computer equipment so I returned the inverter-charger and I will just use the battery with a standard UPS that is designed for computers and will have a fast trip time and better sine wave. Most UPSes take 24 volts. Canadian Tire is out of stock of the exact battery I bought. They have ones with different cold cranking amp ratings. Same AH. Is it ok to mix these? CCA does not really make a difference in a case like this right? I can wait and hope they will restock the exact battery I bought, but chances are this changes all the time. Worse case scenario I may just buy 2 more and keep the current one for some other use.

mpoulton 05-08-2012 07:39 PM

Batteries used in series need to be identical. Any difference at all in cell characteristics can result in a charge imbalance where one battery is undercharged and the other overcharged. This will reduce the available stored energy greatly and damage the batteries. Definitely use completely identical batteries, and make sure they are both completely charged in parallel before you connect them in series.

frenchelectrican 05-08-2012 11:53 PM

Quote:

Originally Posted by mpoulton (Post 917463)
Batteries used in series need to be identical. Any difference at all in cell characteristics can result in a charge imbalance where one battery is undercharged and the other overcharged. This will reduce the available stored energy greatly and damage the batteries. Definitely use completely identical batteries, and make sure they are both completely charged in parallel before you connect them in series.

He did hit right on the spot in series connectioned battery system they have to be identical all the way to get the best performace.

If you have any differnt size the perforamce will really affect is the smaller battery that will get the most of abuse so that why keep both at the same size.

Merci,
Marc

curiousB 05-09-2012 09:25 AM

Quote:

Originally Posted by Red Squirrel (Post 917457)
Turns out the inverter-charger's switch time is too slow for use with computer equipment so I returned the inverter-charger


??? There typically is no switch time. UPS's generally always run from the DC to AC inverter. They are not switched in and out. The chain is as follows:

AC in to DC charger, DC Charger to Battery, Battery to inverter, inverter to 120VAC out.

Since the inverter is always sourcing AC power there is no switching. Rather the only aspect to to this is whether the battery is being charged or not. By doing it this way the AC is always present for the computer load.

As for batteries they should be the same or the charger will over charge on bank and under charge the other. They must be deep cycle batteries as well. Marine battery can mean many things. Be sure you are buying a deep cycle marine battery. Also these charging batteries exhaust gasses (the flooded cell type more specifically) while charging so put them somewhere appropriate so you don't create an explosion hazard.

zappa 05-09-2012 10:54 AM

Quote:

Originally Posted by curiousB (Post 917784)
??? There typically is no switch time. UPS's generally always run from the DC to AC inverter. They are not switched in and out.

Maybe there are both kinds but the 5 or 6 different APC models (older) that we have at work run on utility AC and switch over to inverter by a simple relay. APC claims that the switch time is much faster than needed for computers.

Red Squirrel 05-09-2012 04:19 PM

Most lower cost UPSes have a fast relay that switches to inverter, as the inverter is probably not designed for continuous use. The one I bought took a good 2 seconds to switch. Others are like <30ms. A computer UPS is designed for sensitive equipment so it's usually very fast, more like <10ms. A computer PSU has to have a hold time higher than the switch time of the UPS or it will die during the transfer.

There are online UPS which works like: rectifier -> batteries -> inverter. Similar to a telco setup. These are more expensive. Idealy I'd like to create that setup eventually but it gets trickier as you need to get the battery charging setup dead on or it will kill the batteries, it also produces more hydrogen as the batteries are constantly being charged. Need proper float voltage, add water, etc...

For now I will just try to find the matching battery and use a UPS. They usually run on 24 volts.

Speaking of UPSes, I have another question. So if I have 2 12v 100ah batteries in serries that gives me 24v and 100ah. Will that last twice as long as 1 12v battery at 100ah for a UPS that only takes 12v batteries?

So say I have a load at 120v will it last twice as long on a 24v UPS than a 12v one with the same batteries? (12v one having 1 100ah battery and the 24v one having 2 100ah batteries in serries)

Since with two 12v batteries in serries they are only handling half the amps as the setup with a single one, correct?

So a 500w load will require 41 amps at 12vdc while it will only require 20 amps at 24vdc. Is this right?

So with 2 100ah batteries in serries (24v) converting to 120v a 500w AC load should last about 5 hours, does that sound right? Not taking into account efficiency loss mind you, so maybe more like 3-4 hours real life. Double that if I add another set of batteries in paralell.

ddawg16 05-09-2012 05:26 PM

I hate to bust some bubbles...but it's really not a big issue if the batteries are different when wired in series.
However, charging them (if gelcell or Lead Acid) takes extra precautions...you have to limit the voltage at each battery....Li and NiCad charge just fine in series.

One of the charcteristics of a battery is the internal resistance....this is what pretty much drives the CCA rating.

Yes, it is desirable to have them the same for the best performance...but if they both have the same AH (amp hour) rating...unless your running the batteries near their CCA rating, it's not going to be an issue.

Lets assume that one battery goes dead early....the only thing that really happens is that the voltage drops off so the total voltage to the load is reduced. It's no different than what happens in a flashlight with 2 or more batteries. The dead battery just ends up being a pass through for the current....

Red...regarding your calculations....your in the ball park....

A 100AH battery should be able to supply 100 amps for 1 hr...but note, your voltage will drop below 12v...so your total watts are not going to be the same. In the case of NiCad batteries...the typical spec for a AA is 100mah and the limit is considered reached when the voltage drops below 0.9Vdc. On a Lead acid...I think the voltage is 10.5Vdc.

Red Squirrel 05-09-2012 06:45 PM

Good to know. If I have no luck in sourcing the same model battery I will just try to get as close as I can while keeping same AH rating. They have a bunch of different models of the same brand, but different CCA rating, like 650 instead of 730 for example.

Their site shows they have the one I have in stock so I've been trying to call them to see if they have it and it's just not displayed, or if their site is out of date. No luck reaching anyone yet. I'll probably wait till the weekend to go there again to see if they got more stock. I'm hoping I can find the same battery as I'll just feel better knowing they're exactly the same.

mpoulton 05-09-2012 07:17 PM

Quote:

Originally Posted by ddawg16 (Post 918068)
I hate to bust some bubbles...but it's really not a big issue if the batteries are different when wired in series.
However, charging them (if gelcell or Lead Acid) takes extra precautions...you have to limit the voltage at each battery....Li and NiCad charge just fine in series.

That's like saying that it's not a big issue to fly an airplane, you just have to keep the wings level and take extra precautions when landing. To say that any batteries "charge just fine in series" is a huge oversimplification. IDENTICAL cells of many battery types (including lead acid) charge just fine in series, as long as they are all equally charged before being placed in series. Every 12V lead acid battery is a series of 6 cells anyways - doubling that to 12 IDENTICAL cells for 24V is no problem at all. But using cells with different characteristics (and different internal resistance is a highly relevant characteristic, since it is directly related to coulombic efficiency) will result in a slowly increasing imbalance in cell voltage over time. It's not going to work out well at all in the long run. Using dissimilar cells in series when connected to a cheap UPS, which has an internal charger that is designed for the higher cell voltage of a gel cell battery, is likely to result in overcharging and destruction of one of these two deep-cycle batteries fairly quickly. That failure will be followed quickly by the overcharging failure of the other battery.

And lithium ion cells do not charge well in series. They are used in series packs all the time, but with very careful control of individual cell voltage - often requiring separate charge monitoring leads to each cell in the pack.

Red Squirrel 05-11-2012 06:58 PM

I ended up finding the same battery, so I'm good now. Turns out they had some in the back. I asked about the longetivity8 of a specific product number and the guy told me that when they do change, it's usually the exact same battery just changing to another name/brand but internally it's the same. So they can go to the manufacturer and find out what the match is. Good to know as I'm sticking to two for now and may buy two more in the future if I want to extend the run time.


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