Growing a battery bank

[tjspears]

I looked around a bit and did not find a direct answer to this.

I want to plan for growing the battery bank. I figure that that is one of the things that it is cost effective to start small and increase capacity over time, unlike the inverter.

The plan is to start with four Trojan T-105's configured in a 24v bank. This is generation A, the following year or later as cash is available Adding four more T-105's generation B

Doing this is wrong according to everything I understand.

Year 1



(+)                 (-)

---[A][A][A][A]---

Year 2



(+)                 (-)

---[A][A][A][A]---

|----|

What about doing this

Year 1



(+)                 (-)

---[A][A][A][A]---

Year 2



(+)                 (-)

---[A][A]---

|--[A][A]--|

Then later if need I could add eight more generation C



(+)                 (-)

---[A][C][C]---

|--[A][C][C]--|

|--[A][C][C]--|

|--[A][C][C]--|

Does this work?

Thanks,

--James.

[AbyssUnderground]

Every time you add a new "bank" of new batteries, you should not connect them to the existing bank because old and new do not mix very well. The old batteries will often kill the new ones down to the same standard. If you're going to update your bank, its best to do the whole bank with new batteries in one go. If you'd rather risk the new ones by connecting to the old then do so with caution.

[alancorey]

I think I've seen the advice that if you have to do it, you shouldn't do it by putting new in series with old.  It's nice not to do it at all, but maybe not realistic unless you can sell off the 1 year old batteries and buy all new.  I'm thinking of doing about the same thing, with about the same batteries.

This:

(+)                 (-)

---[A][A][A][A]---

|----|

Is better than this:

(+)                 (-)

---[A][A]---

|--[A][A]--|

Even better would be to set up a separate system with the new batteries.

  Alan

[Nando]

There are too many types of ideas to install batteries when age or type are mixed.

The basic principle is to have the battery banks with equal voltage at the end of each bank.

Some batteries age and may have a lower voltage.

Since you are adding a complete set, then the best arrangement is for you to use

the scheme

---[A][A]---

---[A][A]---

This way both banks will have the same voltage

Most places do not do such arrangement because it may be hard to keep track of the old and new batteries, do proper marking will do the job even if more ae added in the strings.

A third string presents an unbalance, therefore two strings should be added instead of one to keep the balance.

This type of arrangement was the one used in the family farms for many decades.

Nando

[veewee77]

Actually, [A][A] is not a good arrangement.

The [A] ones are used and likely lower, won't take as much charge and etc and will cause the whole string to take more current to bring the string up to float voltage which will cook the ones faster.

If it were me, I would do it like this:

Year one:

[A][A][A][A]  220 AH capacity if they are Sam's Club Golf car batteries,

              use only 44 AH from them, or 1056 watt-hours.

Year two:

 440 AH, use only 88 AH from them

[A][A][A][A]  or 2112 watt-hours.

Year three:

[C][C][C][C]  660 AH, use only 132 AH from them

 or 3168 watt-hours.

[A][A][A][A]

Year four:

[D][D][D][D]  880 AH, use only 176 AH from them.

[C][C][C][C]  or 4224 watt-hours.



[A][A][A][A]

And just take very good care of the batteries as far as water levels and discharge. NEVER discharge to lower than 80% SOC. This means LEAVE 80% In them.

This is based on a 24 volt system, and providing you have enough input to make up the watt-hours used each day. Of course if you use less than these watt-hour figures, you will never draw them below 80% and therefore, will not kill them in an untimely manner, no matter what. Just make sure that if you use considerably less than these figures, don't put in too many battery strings because they do need to be exercised.  If you only use 300 watt-hours at a time from a 880AH bank, they will not get exercised properly and that is just about as bad as draining them too deep.

Key point here. . . don't put more battery in than you need on an average and keep them up properly for the longest life.

After the fourth year, just start rotating them out, oldest first. Use the four year batteries as cores on the new set you are replacing them with.  Do this and you should get 4 years from each set of batteries, even if you just use the el-cheapo Sam's Club golf car batteries. If you never drain them too far, and keep the water levels up religiously, and do a good equalizing charge once a month or at least every other month (to keep the electrolyte stirred) they will last quite some time.

JMHO - YMMV

Doug

[SamoaPower]

As frequently happens, we see a number of divergent views on battery use and care. I think they are the most mis-understood elements in RE systems.

I agree with the majority here that in a series string, best results are obtained from matched individuals so that keeping a purchased set (hopefully from the same lot number) together during their lifetime is the way to go.

Also, I would keep each set of a different type or age as a separate bank to be charged independently unless separate charge controllers are used for each bank.

One point that is often neglected is the decreasing capacity with age and use. We often see people here talking about aged or used batteries and quoting the new name plate capacity as representative of what they have. This is never the actual case.

Batteries start to lose capacity as soon as they are cycled. Major pertinent factors are cycling frequency, depth of discharge, charge algorithm used and temperature. A rough, rule-of-thumb estimate for well cared for batteries is 10% per year. It can be a lot more if neglected.

This capacity loss is why I would not mix batteries of a different age in a series string.

[Nando]

After re-thinking this message, I see that it is best to keep each battery group by age and not mixing them, since age reduces charging capacity, therefore you may be reducing the capacity of the mixed ( by age ) bank.

Nando

[cyplesma]

is this where a battery isolator would come in handy.

I don't know how hard it would be to find one for 24 volt or how cheap it would be but if you used an isolator for the two banks then at least you could charge them with same charge controller and use them for different zones.

yes / no?

[cyplesma]

of course i believe you would need to add two isolators when you go to add your third bank, and then add three more isolators when you go to add your fourth bank

[SamoaPower]

Unfortunately, isolators wouldn't solve the basic problem of two or more banks of different capacities requiring different charge profiles. Which bank does a single charge controller sense from?

The usual diode isolator only provides isolation during discharge.

[alancorey]

Whew, I thought I'd gotten it backwards after seeing Nando's post.  I think of them in terms of the older batteries having a higher impedance or internal resistance, so putting different resistances in series causes uneven distribution of voltage.  Putting them in parallel just means one charges first but they eventually get the same voltage.

For anyone that missed the parallel thread, see Tom W's post on almost the same subject at http://www.fieldlines.com/story/2007/6/12/162051/150

I like the connecting them together through a headlight bulb idea.

  Alan

[poleframer]

I set up two battery banks,one for my house, one for my shop, with two invereters as follows- I have an ARGO FET isolator from JackRabbit,two traceC35 controllers, two diodes, and a dump load. I'm on hydro. Charge goes to isolater input,output1 to C35 in solar mode charging the house batteries. Output2 to two diodes (1.5 voltage drop), to shop batteries controlled by C35 in diversion mode.

No charge to shop batteries till the house batteries pass the absorbtion stage,then input rises to 15+ volts, then charge to the shop batts untill charge finally diverts.

I have one set of new batts for the house(4 L16s), and two older sets of batts for the shop (8 L16s).  

I'm not sure if some variation would work with one inverter. The price on the FET isolator wasnt bad ($170 or so) almost no voltage drop. I do take a 6% loss to the shop batts due to the diodes, but they keep the new house batts prioritized, and I dump the excess anyway.

[cyplesma]

I have an isolator I bought for a different project, but what I remember of how it works is (in a car hookup of one alternator to charge two batteries) the isolator determines which battery gets the amps, when one is full the isolator automatically transfers the full amps to the other battery, now if both batts are full how do you know to start sending the amps to a dump load, that would be of a concern

[cyplesma]

This is the type of isolator I'm talking about. This one is for 12v but I believe they make them for 24 volts.

http://cgi.ebay.com/ebaymotors/DUAL-MULTI-2-BATTERY-ISOLATOR-CAR-TRUCK-BOAT-140-AMP_W0QQcmdZViewItem
QQcategoryZ33573QQihZ017QQitemZ270128506756QQrdZ1

[poleframer]

When the 1st bank is charged the controller shuts off the current to those batteries, when the second bank is charged current is diverted. At any point either bank will restart charging when needed, with bank one prioritized.

You could also do this with battery combiners- check out the balmar duocharge.

 http://www.jackrabbitmarine.com/Detail.bok?no=2229

[cyplesma]

interesting

it looks like that toy could even be used as a dump controller.

A little on the pricey side, but they seem to do a lot of stuff for boats, and boating always has been an expensive lifestyle.

8 )