2 x different AH AGM batteries (1 Viewer)

[SteveandSueJ]

May i ask for advice please. I have an AGM battery (Varta Stop/Start) from my car (surplus to requirements) that is 68AH. I currently have a single 110AH AGM battery in my MH as the Hab battery which works fine. Can I connect them together in Parallel to increase AH output without issue? Thanks in advance.

 

[The Wino]

They're really supposed to be a matched pair in terms of age type and capacity.

 

[SteveandSueJ]

I have read up on various forums and get mixed replies. Some say as long as noth are the same chemistry (AGM) then fine if in parallel. I am not sure, so keen to hear from anyone who definitively knows or who has done/doing this. Thanks...

^{_{Subscribers  do not see these advertisements}}

 

[Jev88]

When linked the charger 'sees' one big battery. But personally I wouldn't do it as my little brain doesn't understand how the bigger AH battery gets fully charged without pushing too much to the smaller one.

 

[Sneds]

Both would act as if one, electrically it will be just one battery.
Problem here is the starter battery is not designed to be a leisure battery, the starter battery is designed to give a lot of current for a short space of time (starting the engine) and a leisure battery is designed to give lower current for a longer period of time (lights on, watching tv etc)
Your starter battery would drain the life out of the leisure battery as they are not matched.
If you want two leisure batteries to double your available power buy two new ones, an old one and a new one will knacker the new one.
Hope that makes sense

 

[pappajohn]

When linked the charger 'sees' one big battery. But personally I wouldn't do it as my little brain doesn't understand how the bigger AH battery gets fully charged without pushing too much to the smaller one.

The little one sees full charge before the big one which absorbs the remaining charge.
the batteries will only take what they need.
Imagine a 1ltr bowl inside of which is a 500ml bowl.
With a 1.5ltr jug start filling both bowls equally and when the small one is full it will overflow filling the large bowl to capacity.

^{_{Subscribers  do not see these advertisements}}

 

[SteveandSueJ]

The little one sees full charge before the big one which absorbs the remaining charge.
the batteries will only take what they need.
Imagine a 1ltr bowl inside of which is a 500ml bowl.
With a 1.5ltr jug start filling both bowls equally and when the small one is full it will overflow filling the large bowl to capacity.

Sounds as though you believe my suggestion is viable, to use both in parallel??

 

[pappajohn]

Sounds as though you believe my suggestion is viable, to use both in parallel??

Yep.....depending on age though.
Batteries lose capacity with age so that 110ah could be 90ah today so recommended they are similar age to get the true capacity but they will work regardless

 

[SteveandSueJ]

Yep.....depending on age though.
Batteries lose capacity with age so that 110ah could be 90ah today so recommended they are similar age to get the true capacity but they will work regardless

Thank you...

^{_{Subscribers  do not see these advertisements}}

 

[autorouter]

I think it's worth a try to see if the two batteries work together OK when connected in parallel. But if it doesn't work too well, there are alternatives.

Even if you had two completely different batteries, say an AGM and a lithium, you could still use both. You could have a changeover switch that switched the habitation electrics between the two batteries. For charging you could have two mains chargers, one for each battery. Easier than switching the charging profile every time you switch batteries.

 

[SteveandSueJ]

I think it's worth a try to see if the two batteries work together OK when connected in parallel. But if it doesn't work too well, there are alternatives.

Even if you had two completely different batteries, say an AGM and a lithium, you could still use both. You could have a changeover switch that switched the habitation electrics between the two batteries. For charging you could have two mains chargers, one for each battery. Easier than switching the charging profile every time you switch batteries.

Good plan, thank you...

 

[nicholsong]

OR Just carry the starter battery, not connected, as a back-up if the main battery gets low or fails, with a switch to connect it if needed. Switch to the spare occasionally to keep it charged.

I am thinking of putting in a LiFePo4 and keeping an existing L/A as a back-p.

This is the cheapskate solution.

^{_{Subscribers  do not see these advertisements}}

 

[autorouter]

OR Just carry the starter battery, not connected, as a back-up if the main battery gets low or fails, with a switch to connect it if needed. Switch to the spare occasionally to keep it charged.

I am thinking of putting in a LiFePo4 and keeping an existing L/A as a back-p.

That's known as a hybrid solution, and is a definite possibility. You may find this thread of interest:

LiFePO4 (Lithium) and Lead Acid Hybrid Arrangement

Just for interest (I found it interesting anyway). I recently helped a boat owner debug a charging issue. He has a 300AH LiFePO4 battery with Eve cells and a Daly 150A BMS. A Daly 150A can accept a charge of 50% of its capacity, so 75A in this case. He has the LiFePO4 battery connected to a...

www.motorhomefun.co.uk

 

[SteveandSueJ]

That's known as a hybrid solution, and is a definite possibility. You may find this thread of interest:

LiFePO4 (Lithium) and Lead Acid Hybrid Arrangement

Just for interest (I found it interesting anyway). I recently helped a boat owner debug a charging issue. He has a 300AH LiFePO4 battery with Eve cells and a Daly 150A BMS. A Daly 150A can accept a charge of 50% of its capacity, so 75A in this case. He has the LiFePO4 battery connected to a...

www.motorhomefun.co.uk

Thank you... I think the idea of the 2nd battery in parallel but on a switch, to enable when needed seems a good plan. Switching equally allows controlled charging.. lots to consider....

 

[Marie-Claire]

Is that the same with lithium , we are getting a new van in june which comes with a 100ah lithium, I will have 2 x 100 lithium from the old van , 100ah each and 18months old, could I wire them all in parallel to give a 300ah total?

^{_{Subscribers  do not see these advertisements}}

 

[The Wino]

Is that the same with lithium , we are getting a new van in june which comes with a 100ah lithium, I will have 2 x 100 lithium from the old van , 100ah each and 18months old, could I wire them all in parallel to give a 300ah total?

I think if it was me I'd use the 18 month old ones that you know work together and sell the new one as I don't know about the compatibility of different makes of lithium with different BMS. It would however make any warranty issues " interesting" and you would need to make sure the B2B are charger are up to it

 

[SteveandSueJ]

Is that the same with lithium , we are getting a new van in june which comes with a 100ah lithium, I will have 2 x 100 lithium from the old van , 100ah each and 18months old, could I wire them all in parallel to give a 300ah total?

Good question. From what I got back from my question, only issue might be age and condition of batteries in case of degradation. At least yours are all of the same ampage. I would mix yours in parallel, not mine as differing ampage. The experts will be along soon I suspect.

 

[Marie-Claire]

I am trying to understand the b2b , we are fairly light users, around 30ah per night if off grid. How is the b2b affected if the recharge required is to 1 battery or spread over 3? As it's still 30ah required?

^{_{Subscribers  do not see these advertisements}}

 

[The Wino]

I am trying to understand the b2b , we are fairly light users, around 30ah per night if off grid. How is the b2b affected if the recharge required is to 1 battery or spread over 3? As it's still 30ah required?

I'm no expert but the amount of charge required will be the same the charging rate would differ. I think you need to ask the dealer your new MH is coming from.

 

[Marie-Claire]

It is allready on the questions list, a little understanding helps so as not to get baffled

 

[VXman]

I am trying to understand the b2b , we are fairly light users, around 30ah per night if off grid. How is the b2b affected if the recharge required is to 1 battery or spread over 3? As it's still 30ah required?

It will take longer. It has been suggested that chargers can break because they are not designed to be charging as long as that. 3 batteries will work the charger a lot harder.

^{_{Subscribers  do not see these advertisements}}

 

[Marie-Claire]

Many thanks

 

[autorouter]

A B2B works as a 3-stage charger, taking power from the alternator at whatever voltage it pushes out (12V, 15V, whatever) and changing it into the voltage and amps that the battery needs.

Typically, in the first stage it pushes out its full amps output, while constantly monitoring the voltage as it slowly rises to see if it has reached the target voltage. If there are three batteries, all in about the same state of charge, the amps will be divided about equally between them. If one of them is for some reason in a lower state of charge, it will get more of the amps than the others, but the total will remain the same.

If you want to put back 30Ah that you took from the day before, then a 30A B2B would take about 1 hour to do this, in theory. In practice, it would work at 30A for about 50 minutes, until it put 80% of the charge back, then dial back the amps at a constant voltage until the amps fell to about 3A, which might take 30 minutes or so. It would make no difference whether there was one battery or ten batteries, it would still be putting 30Ah back.