Adding a new lead acid battery in parallel to an old one?

nginx · September 2012

Hi everyone,

I am looking to go off-grid partially with Solar Power. I already have a 3 year old 160AH lead acid battery hooked up to an 1KW inverter which keeps my house powered partially during power outages which are quite frequent where I live.

My battery still seems to be working as good as new despite its age. I want to put a brand new 160AH battery in parallel with the existing one to extend runtime and get me through the night. Is there any cause for concern in doing this?

I have heard before that only brand new batteries should be paralleled. But it doesn't make economic sense to throw away a perfectly good battery. These batteries aren't exactly cheap either.

westbranch · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

mixing batts of different ages is not recommended as they interact between themselves. It could work if you were to isolate the batteries with a Blue Sea 1-2-bothl type marine switch used on boats.http://www.solar-electric.com/basw1300amp.html

Cariboocoot · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

Welcome to the forum.

The problem is that a "perfectly good battery" is not the same as one that still has 100% of its capacity as the new one would. As a battery ages it inevitably loses capacity. The deeper it is discharged, the more cycles it is run through, and the time that passes all take their toll. The trouble is there is no practical way to determine how much the capacity has diminished.

Why it is bad to put an old and new battery together is the same as why it is bad to put two batteries of different capacity together (because that's what you've got): the charge will be wrong for one, the other, or most likely both. Too little for the higher battery, too much for the lower one. The greater this difference is the greater the risk of either frying the old battery (in a possible sudden failure) or cutting years off the life of the new one.

Another matter: do you have enough recharge ability to add more battery capacity? Chances are even if the system was properly designed to recharge one 160 Amp hour 12 Volt it can not do the same for two (320 Amp hours).

Since you are thinking of buying a new battery, needing to expand capacity, and inevitably will have to increase charge capacity it is time to re-evaluate. You probably would be better off buying one new battery of greater capacity (or two 6 Volt units) than adding another of the same on to the existing.

nginx · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

Cariboocoot wrote: »

Welcome to the forum.

The problem is that a "perfectly good battery" is not the same as one that still has 100% of its capacity as the new one would. As a battery ages it inevitably loses capacity. The deeper it is discharged, the more cycles it is run through, and the time that passes all take their toll. The trouble is there is no practical way to determine how much the capacity has diminished.

Why it is bad to put an old and new battery together is the same as why it is bad to put two batteries of different capacity together (because that's what you've got): the charge will be wrong for one, the other, or most likely both. Too little for the higher battery, too much for the lower one. The greater this difference is the greater the risk of either frying the old battery (in a possible sudden failure) or cutting years off the life of the new one.

Another matter: do you have enough recharge ability to add more battery capacity? Chances are even if the system was properly designed to recharge one 160 Amp hour 12 Volt it can not do the same for two (320 Amp hours).

Since you are thinking of buying a new battery, needing to expand capacity, and inevitably will have to increase charge capacity it is time to re-evaluate. You probably would be better off buying one new battery of greater capacity (or two 6 Volt units) than adding another of the same on to the existing.

Thanks a lot for the detailed reply. I can perhaps get one bigger battery but what do you suggest I do with the existing one? I can't just throw away a good battery as that will be a huge waste of money. We have no exchange offers here for old batteries.

My inverter has built in charger to charge the battery from the AC mains. There is no need to upgrade the inverter because 1KW capacity is enough for me. Once I purchase a solar charge controller, I will keep the inverter's built-in AC charger turned off and only use in case of emergencies (cloudy days). I have already made my calculations regarding the size of solar charge controller I need and the size of panel array for my average load to get through the night.

At this point I am just wondering how do people usually expand their systems? Everytime there is a need to expand the system, do people usually throw away all the old batteries and buy new sets? Let's say one battery fails in a 24V series setup after 2 years. Does it mean both batteries have to changed because we can't mix old with new? If one has a 2P2S battery setup and one battery fails, surely its not possible to change all the batteries just to keep the whole set balanced?

Wxboy · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

Like westbranch said get a marine battery switch. Then you can leave the old battery charged up and on standby while you use the new battery on a regular basis. When you have an outage you can drain one battery and then switch to use the other battery. There is also a setting to use both batteries at once. We know you don't want to charge them together but I don't know if it is bad for the batteries to drain them together using the inverter. Someone will be able to answer that question I'm sure.

Cariboocoot · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

nginx wrote: »

Thanks a lot for the detailed reply. I can perhaps get one bigger battery but what do you suggest I do with the existing one? I can't just throw away a good battery as that will be a huge waste of money. We have no exchange offers here for old batteries.

My inverter has built in charger to charge the battery from the AC mains. There is no need to upgrade the inverter because 1KW capacity is enough for me. Once I purchase a solar charge controller, I will keep the inverter's built-in AC charger turned off and only use in case of emergencies (cloudy days). I have already made my calculations regarding the size of solar charge controller I need and the size of panel array for my average load to get through the night.

At this point I am just wondering how do people usually expand their systems? Everytime there is a need to expand the system, do people usually throw away all the old batteries and buy new sets? Let's say one battery fails in a 24V series setup after 2 years. Does it mean both batteries have to changed because we can't mix old with new? If one has a 2P2S battery setup and one battery fails, surely its not possible to change all the batteries just to keep the whole set balanced?

How do people usually expand their systems? They don't. It is one of the most problematic aspects of RE. If you start out knowing you are going to expand you can plan two systems and integrate some of the components of the first into the second. For the most part, unplanned system expansion is an expensive adventure. Hopefully you can time any major upgrades around the need for battery replacement (which is inevitable).

In terms of a single battery failing in the bank, this usually doesn't happen. I will qualify that by mentioning that it did in fact happen to me in 2008, but within one month of buying the new batteries. If they are that new it isn't a problem. It's only when they get aged (by time, cycles, or discharge depth) that it becomes an issue. You can mix old with new, but know in advance that there will be a potential for trouble as I mentioned before.

Westbranch's suggestion of the A-B switch is the most practical way of keeping the old one around 'til there's no more life in it, but you will have to remember to monitor both batteries and make sure each gets regular charging. If you forget and let 'B' sit on its own it will discharge, sulphate, and die.

In answer to Wxboy's question, if you connect the two together for loads only (somewhat difficult to achieve) the battery with the greater charge will try to charge the lower one. At some point being connected together they will stay equal, but it will not last.

niel · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

to make it clear, you can parallel a new battery with your old one, but as soon as you do the new battery will take on the same age and capacity characteristics of the old one. you can also parallel an identical older battery to your present old battery, but whichever is worse will be the standard for all. do you really know the condition of your present battery as it may be getting ready to die in a month or year for all you know?

i don't know your reasoning for the expansion, but i say keep using this one until it's demise even if you drain it beyond the 50% dod point and then get the battery bank and the solar system revamped later.

nginx · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

Thanks guys. I understand the problems now. I don't want to go with the marine battery switch idea as manual monitoring is not possible for me. I guess I will keep using this battery for the time being just to make full use of it. When it dies I will get a couple of big batteries for a 24V series setup with the solar panels.

@niel, I don't have any means to measure the capacity of the battery. Its probably not 160AH anymore due to its age but I reckon its still in very good condition & will easily last another 2 years because during a recent prolonged power outage it lasted around 5 hours before the inverter's safety cut off kicked in. Its hard to say what kind of load it was under cause it varies all the time as people in the house randomly turn on and off bulbs, fans and laptops etc.

To answer your other question, I am expanding because currently my setup is only meant to provide power during power outages which usually don't last longer than 2 hours. The inverter automatically charges the battery by drawing power from AC mains once power returns. The 160AH battery is more than enough for this purpose. But now I want to go off-grid by installing solar panels so I have to make sure my battery bank is large enough to store enough juice during the day to get me through the night. Obviously 160AH is not enough according to our load requirements. Hence the need for expansion.

niel · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

without load testing with special equipment you can only guess at the capacity and i really had not meant for you to measure it as i consider it a moot point being 3yrs old anyway.

again i wasn't really concerned with why the expansion, but if you stated you have more loads or something it wouldn't really make a difference because you want to get the most from what you have now and do the proper expansion later with more pv and maybe even a different cc if you wish.

if you get an outage and it fails to function as you need it to, then you know it's time.

inetdog · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

niel wrote: »

.

if you get an outage and it fails to function as you need it to, then you know it's time.

Actually, that it WAS time a couple of months earlier. :-)

niel · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

inetdog wrote: »

Actually, that it WAS time a couple of months earlier. :-)

really????
"My battery still seems to be working as good as new despite its age."

inetdog · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

niel wrote: »

really????
My battery still seems to be working as good as new despite its age.

Good, and you have not had an outage and found it lacking yet, obviously. And probably won't in two months if you are keeping good track of it now.

niel · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

sorry i fixed my post as that was a quote from the op saying it is working like new so how can you say,
"Actually, that it WAS time a couple of months earlier. :-)"

ggunn · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

nginx wrote: »

Hi everyone,

I am looking to go off-grid partially with Solar Power. I already have a 3 year old 160AH lead acid battery hooked up to an 1KW inverter which keeps my house powered partially during power outages which are quite frequent where I live.

My battery still seems to be working as good as new despite its age. I want to put a brand new 160AH battery in parallel with the existing one to extend runtime and get me through the night. Is there any cause for concern in doing this?

I have heard before that only brand new batteries should be paralleled. But it doesn't make economic sense to throw away a perfectly good battery. These batteries aren't exactly cheap either.

As others have said, you do not want to do this. As batteries age, their internal resistance changes, and this resistance determines the charging current that passes through the battery. With a pair of batteries that have always been in parallel with each other in a good design, i.e. with matched cables, they age more or less identically, so as their charging currents change, they remain the same as each other.

BB. · September 2012

Re: Adding a new lead acid battery in parallel to an old one?

In the end, it will not cause a safety hazard--Just an unknown interaction between old an new batteries. You may end up having to replace the older battery in 3 more years--And it will be unclear if the new battery will last another 6 years, or age faster and die around the time of the older battery.

If you need the extra capacity (and have the solar/genset/charging resources to support a larger battery bank)--It may still be worth it for you to install the new battery and see what happens.

The downside being that the new battery may not last as long or you may get in the cycle of replacing a failing battery every three years instead of both batteries every 6 years (nominal numbers, your actual battery life will probably be different).

With any parallel battery bank setup (and for general solar power system debugging in general), I would highly suggest getting a DC Current Clamp Meter (Sears sells one with DMM funtions that is "good enough" for ~$60).

Once or twice a month, go out to the battery bank and when under load and under charge measure the current into each battery (string) and make sure they are (roughly) sharing the load/charging currents.

When on battery stops "sharing" the current--You need to debug (look for open/shorted cells, as well as bad electrical connections).

I am one of the first people to try and guide people away from mixing batteries--But if you are up for the added maintenance and want to give it a try--Why not.

If this was a "critical" power system--I would highly recommend not mixing battery banks. However, if you can spend the time watching the system and replacing one battery at a time--go ahead.

Note, when you parallel batteries, you should have a fuse/breaker per string to prevent a short on one battery string from being feed by the other string--this does add wiring/costs to parallel battery system--and one of the many reasons why I/we really recommend going to a single string of larger AH batteries rather than paralleling--others include more electrolyte caps to check, more cabling to check, more cells/batteries where something can go wrong, hidden failures that can damage other batteries like shorted/open cells, etc.

-Bill

Adding a new lead acid battery in parallel to an old one?

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