Mixing batteries

[Ogou]

Hi,

I purchased a small UPS normally used for computers. It has a small sealed lead acid battery inside, 12V, 7Ah.  I'm using a small 20W solar panel to recharge the battery.  Connections are made directly in the battery.

I need more power in the battery so I'm planning to buy a bigger battery 12V, 150Amp and connect it in parallel to the battery inside the UPS.

Is it possible to mix battery like that? Both batteries a new.

12V 7Ah and 12V 150Ah

[willib]

i'm not sure if I would parallel the two.

but i might use just the larger one if i needed more power.

[AbyssUnderground]

You would quickly kill the 7Ah SLA if you paralleled it with a bigger battery. It would be better to just use a larger battery rather than the smaller one and the large one. Id also make sure that the charger on the UPS can handle the larger battery and that the inverter can also handle the longer runtimes. Some overheat if used with a larger battery and runs for longer than the specs say it should.

[Jon Miller]

Hi

The size of the internal UPS battery suggest to me that you have a APC 500VA or similar sized UPS.  There maybe an issue with the internal testing the UPS does on start up if you run the ups exclusively from the 150Ah battery.  The UPS expects to see a 7ah SLA battery and may chuck the overload light on and fail to work.  

I would suggest trying the ups with the 150ah battery but not in parallel as the 7ah battery will not last long due to the increase in potential current and the typical low internal resistance of SLA batteries.  maybe a switch over switch is a possible option, start the UPS up on the 7ah then switch over to the 150ah once it is up and running

good luck with it

 

[AbyssUnderground]

I don't think a UPS can distinguish between battery sizes. The one it might throw a wobbler on is if it doesn't charge the battery in "x" amount of hours to "xx" volts. This would cause a "check battery" condition and might prevent it working.

[Nando]

Heck !! Guys do not complicate life to such extreme.

The 7Ah is just a very small if compared to the 150 Ah battery, it just follows the larger one behavior, nothing else.

If left inside, it just serves at a larger capacitor for the length of cable between the 150 ah and the UPS.

Make sure that the equalization voltage is close to each other for best life of the battery and maybe a few millivolts below the larger 150 AH battery bank.

The UPS seems to be in the low side around 500 or so watts, so make sure that the electronics do go well with the larger battery.

Nando

[whatsnext]

I agree with Nando on this. The only potential problem I see is that the UPS maker won't have designed much heat sinking on the charger portion knowing that it would charge the 7Ah battery quickly. I've upsized batteries on UPSs numerous times without a problem. I wouldn't even bother sizing the cables to the battery because they really need to be sized to the UPS's draw which they already are.

John......

[ghurd]

Good point guys.

A 7AH and a 150AH.  So what.

The 7AH will not reach 13.7 before the 150AH when charging.

The 7AH will not reach 12.2V before the 150AH when discharging.

The problem I do see is needing more power than a 7AH battery, while charging with a 20W panel.  

The self discharge of the battery will take an hour and 15 minutes per day of the panel's (insolation) output.  

It needs a bigger than 20W panel, and a bigger than 7AH battery.

G-

[Ogou]

Thanks a lot to every body.

I just want to add, I don't want to remove the 12V 7Ah battery because it gives me the "portable" option.  So when I need little power it works fine but When I need 5-6 hours, I will add the bigger battery.

Also th 20W panel, it too small for a 150Ah battery you are right but I'm not going to use it everyday. So the panel can recharge the battery in 1 week.

[Bruce S]

Ogou;

   I do something similar for our chest style freezer. The UPS is rather small and I modifed the side panel where the battery is by putting battery lugs on it to connect the external batteries. This way I can take them off and recharge them or use them else where when I need to.

One of this things you'll find is that at anything more then 3 times the current internal battery rating and the internal charger will have problems keeping it charged. You'll actually be able to hear it kick in and out as it constantly tries to charge it up if gets gotten too low due to power outage or something.

The top of the UPS will indeed get hot to the touch, which is due to over driving the battery charger.

With respect to the usage of the larger battery sets. I have not had any problems connecting 7 external SLA together during our past power outage. Make sure to use high quality mulit-strand wire connections and you will be just fine. After our power was back on, I disconnected my externals and put them on the solar charger.

You will need to be sure and not allow the UPS charging circuit to charge through the solar panel:-) but I'm sure you've already thought of this.

What uses do you have in mind for this unit.

To let you know, mine isn't setup the be able to start the Freezer, just keep it running . I have found that while the mains is on the freezer doesn't have any problems starting while on the UPS, and will keep running when I give it a false outage ( unplugged from mains)with no probelms , this included listening to it start the compressor and such.

If it for something else such as lighting then you shouldn't have any problems.

We even had TV on another similar UPS for several hours each day, by merely having the internal battery stay healty by switching out external batteries.

Let us know what your plans are, there are several who went through the Midwest outages that can give real-life accounts.

Above all HAVE FUN!!

Bruce S

 

[Ogou]

Thanks Bruce,

I don't need the UPS to start a freezer. The appliances I want to use it for are much smaller : laptop computer, light and sometime a TV.  So I'm not too worry about overlaoding the UPS.

Now the problem is charging the 150Ah battery. With a 20W panels, it will take several weeks. So the 150Ah battery, will be use for extra power.

I'm thinking about this unit for the Third World when there the grid does not exist.

Thanks

Ogou

[ghurd]

"thinking about this unit for the Third World when there the grid does not exist"

That changes everything.

Everything.

[Dzisko]

You are right. Internal resistance of both batteries (in case they are in a good condition) is extremely low, so in case they have different voltage there can flow impredictable big current, of course small battery can be damaged with this big current.

What I think - we have to try slightly touch connectors firstly and in case sparks are small enough, connect two batteries. In case sparks are big - then connect through resistance, car bulb, for example, then charge/discharge batteries each of other, then check again and in case sparks will be small, then parallel batteries.

Most of UPSes check only 2 things in batteries - voltage (it have to be big enough, in case it less then predetermined, UPS will not start - you have firstly charge batteries with external charger) and internal resistance of battery - it have to be small enough. If it is extremely small in case of a big battery UPS will simply think it is extremely good battery.

Most of UPSes will burn out in case of long invertors loading - they have not a good cooling system, so if you decide to use them as a longterm power supply, you have increase heatsinks and install coolers if they are absent (as a rule low power UPSes have not any coolers).