Adding a Battery

[MsNomer]

I have one 100Ah AGM battery in the van. It is approaching a year old and has actually been in use about half that time. I want to add another identical battery in parallel, but I read that batteries in parallel must also be the same age. How strict is this age requirement?

[markopolo]

It is a "Best Practice". Same age, type & capacity.

For me, I'll mix & match based on resting voltages. The resting voltages have to be similar. I wouldn't mix a battery that rests at 12.60V with one that rests at 12.70V. I like them to be closer than that.

[ManWonder]

my new (to me) rig came with 2 brand new interstate srm-27 batteries.

#1 - I am unable to ascertain the amp hours - I have looked up the specs and all I see is a range 70 to 100. So what do I go with?

#2 - If I want to augment these two, can I add AGMs?

TIA -- this forum is really great, really helpful to newbies

[BBQ]

Originally Posted by MsNomer

I have one 100Ah AGM battery in the van. It is approaching a year old and has actually been in use about half that time. I want to add another identical battery in parallel, but I read that batteries in parallel must also be the same age. How strict is this age requirement?

Adding a new battery to a year old battery is OK.

What the Best Practice allude to is to have the batteries having similar charging and discharging characteristics. You do not want one battery that is fully charged, while the other is struggling to keep up.

You should have no problem adding to a battery that is only one year old.

[BBQ]

Originally Posted by ManWonder

my new (to me) rig came with 2 brand new interstate srm-27 batteries.

#1 - I am unable to ascertain the amp hours - I have looked up the specs and all I see is a range 70 to 100. So what do I go with?

::

What is the product code on the battery?

[booster]

Take a look at this document:

http://http://www.interstatedealers.com/pdf/200004.pdf

The "approximate amp hours at ampere load" chart shows 19.4 hours at 5 amps, which would make it a very close comparison point to the normal 20 hour rating. 19.4 hours at 5 amps is 97 amp hours.

It is a marine starting style battery, so it is not a true deep cycle.

You can't mix AGM and wet cells unless they were put on separate banks and switched between them.