battery low trouble?

[Anthony A.]

Ok so i re-emailed tech support to ask if its possible to damage anything using the 2 x 12ah batteries in parallel and was told that they have not tested this so can only advise on their manual recommendation which is 2 x 7ah. So now im at a roadblock. I would really like to keep both batteries connected if it doesnt damage anything... how can i determine this?

 

[RAL]

Most SLA battery charging circuits charge the battery using a current that is some fraction of the maximum capacity of the battery.  That maximum capacity is assumed in the design of the charger, since there is no way to determine it from the batteries that are connected.  The charger deliberately limits the current to the battery to optimize the recharging of the battery.  
 
By connecting a larger battery, it is unlikely that you can damage the charging circuit.  But the larger capacity will mean that it will take longer to recharge the batteries.
 
One way to verify this would be to connect an ammeter between the battery and the charger while the battery is charging, and measure the current first with a 7 Ah battery and then with the larger battery.   If the both currents are about the same, that would be a good sign that it will work ok.  Of course, it means taking a risk by connecting the larger battery to make that measurement.

 

[Desert_AIP]

You could safely connect ONE 7Ah battery or ONE 12Ah battery to do the test since the charger is rated for 14Ah.

 

[RAL]

You could safely connect ONE 7Ah battery or ONE 12Ah battery to do the test since the charger is rated for 14Ah.

 
Yes!  Good suggestion!

 

[Del91574]

@ RAL
Your theories are sound, however how the charging circuits function is completely different. They have a duty cycle rating on the equipment and there is some basic intelligence involved.
 
Yes, a larger battery will take longer to charge back up from being discharged, however the limiting factor is the rectifier and AC transformer of the panel, as well as the power output of the charging circuit itself. It's not about maintaining the charge of the battery, but bringing the battery back up from being discharged and running the system. Too great of a load you're going to overpower the charging circuit and start burning the electronics. The charging circuits are not dynamic and they don't monitor the voltage coming from the connected battery and switch over to a maintenance charge if the battery has the correct voltage...nor do they do an amp test on them. At best, the panel does a voltage drop test periodically (which is going to be skewed by the larger battery, so maintenance will need to be planned to replace the battery prior to it failing a test).
 
The larger equipment I deal with have settings for the battery <26 aH, and >55 aH as examples....in addition to disabling the charging circuit. There's a reason for those settings.
 
I have a slightly used Vista 128 that the installer decided to run a 26 aH battery on if you want to see what happens to a burg panel with too great of a load on the charging circuit...it's not pretty.

 

[Anthony A.]

Wow, okay i think im gonna remove tbe 2 x 12ah batteries in parallel. Not worth the risk imo. So its either keep just a single 12ah or go with 2 x 7ah. Which is the safer choice or are they both equally fine?

 

[Desert_AIP]

2x7 gives you 24 hour backup.

The 12 may be a bit shorter duration.

In a multi-battery setup if one battery goes bad, it will drag down the other with it.
But a bad cell in a single battery will drag the others down too.

In the end, six of one, half dozen of the other.
I'd go with the largest capacity.

 

[Del91574]

*The largest capacity listed as being supported (and tested) by the manufacturer without guessing.
 
A single 12 aH is not much different to the panel than 2-6aH or 2-7aH's....but going above what is listed as a known and tested combination would not be recommended.