Battery questions

trkarl · January 2009

My bank is almost a year old. It is usually cycled between 20 to 30% dod each night. When I first got it I equalized every month and kept records of every cell. I would also check each cell with the hydrometer periodically throughout the month. Only maybe 4 times did it discharge to about 65% dod.

Twice I found a cell that would begin to get weaker from sg. readings. I would then move that battery to the position of a strong battery and soon it would be back to normal. There was always one particular cell that the sg reading would always be about 20 points below the rest during daily charging. It would get to around 10 pts of the others during eq. The cells seemed to stabilize so I quit checking every cell every month.

Last night after 3 hrs of eq I decided to check every cell, which I had not done for a couple of months, and found one that was 50 pts below the rest of them. This cell was fine before. So my questions to the battery gurus out there are:

How often do you eq?
How often do you check the sg with a hydrometer?
Do you rotate or move your batteries around in each string and if so how often?

I have a desulphator permanently attached to the bank. All the interconnects are the same length.

Tim K

niel · January 2009

Re: Battery questions

that does not sound good and if your sg readings are correct (possible bad hydrometer), then suspect something to be going on either with the wires/connections or the charging system's rate of charge may be too low or both. can you give us more info on the setup you have, the specifications of your batteries and pv system along with wire gauge used? while you are at it take voltage readings of the batteries after about 2 hours from the last charge or use (for at rest readings) in addition to the sg readings for each battery and post that data here.

peterako · January 2009

Re: Battery questions

Hi there i am from Greece and i have also a 48V bank.

Your problem sounds to me more a problem from corrosion from your connections.
If you move the cell you reconnect and temp. the corrosion is away.

clean all connections and grease using proper grease for battery's or spray.
also clean the top from the battery's using clean water, there is always a spray from acid there.

Greetings from Greece

trkarl · January 2009

Re: Battery questions

My system setup:

Array 1) 12 200 watt BP solar panels = 2400 watts wire 3 strings of 4 panels.
FLEXmax 80 controller
Array 2) 4 90 watt and 2 85 watt BP panels = 530 watts wire 1 string.
MX60 controller
Cables from each controller output is 4 awg welding cable.

Battery bank 8 Trojan T-125 batteries for 48 volts 240 ahrs.
Interconnects are 2/0 welding cable.
Inverter cables are 2/0 welding cable.

I have the absorption set point at 61.3 volts on the controllers. It usually is at the absorption stage between 2 to 4 hours each day.
When equalizing I do it with the inverter for about 3 to 4 hrs at 63 volts.
By then the sg of the batteries usually quits rising. Also the batteries do not get above 100 degrees even when the garage is 95 degrees as it is in the summer.

As far as corrosion there is virtually none as all connections are coated with petroleum jelly. I check weekly and as soon as it is found I clean the connection and recoat it.

Yesterday I equalized for about 6 hours and quit at 7 p.m.
Turned the system completely off.
This morning at 10 a.m. I took readings as follows:

Battery string voltage 51.1

The hydrometer is a Freas HD98C with the thermometer.

I numbered the cells when I first got the batteries so I could keep records.
Specific gravity readings:

Battery A) cell 1) 1.290 cell 2) 1.290 cell 3) 1.295

Battery

cell 4) 1.300 cell 5) 1.300 cell 6) 1.300

Battery C) cell 7) 1.290 cell 8 ) 1.295 cell 9) 1.290

Battery D) cell 10) 1.290 cell 11) 1.305 cell 12) 1.295

Battery E) cell 13) 1.290 cell 14) 1.300 cell 15) 1.300

Battery F) cell 16) 1.300 cell 17) 1.300 cell 18 ) 1.285 This cell has always been lower than the others since I got the batteries.

Battery G) cell 19) 1.295 cell 20) 1.300 cell 21) 1.295

Battery H) cell 22) 1.295 cell 23) 1.295 cell 24) 1.245 The problem cell.

Cell 24 is where the negative connection to the inverter is attached. This is why I am wondering if they should be rotated in the string through out the year.

The last eq records I have for the cell is 1.270, 1.280, 1.280, 1.280, 1.285, 1.290
All the cells show similar rises in sg over the time the records were kept.

Maybe it got contaminated or is defective. Any thoughts?

icarus · January 2009

Re: Battery questions

Regardless of the proper answer to your question,,, I want to compliment you on your superb post. It seems all the relevant information is there is one easy to follow posting. The only other thing I might ask is, how are the batteries wired? The configuration of the wiring can have an effect on cell condition. I no experience with 48 volt wiring so I will let someone else chime in.

Also it is pretty cool that you have kept the log of you batteries.

My intuition is that you should rotate the batteries over time.

Tony

trkarl · January 2009

Re: Battery questions

Hi Tony,

The batteries are wired in a single string, 8 batteries x 6 volts = 48 volts. I went with 48 volts to eliminate parallel strings with high amp hour capacity. I want to get Surrettes but I figured I would start with the golf cart batteries so if I kill them before I figure everything out it wouldn't kill my wallet.

The bank I want to end up with is the surrette 6cs17ps or 6cs21ps which would give me plenty of reserve capacity.

Also I wanted to keep the cables no more than 2/0 in size. 48v requires 1/4 the amps for a give load than 12v or 1/2 that of 24v.

Tim K

niel · January 2009

Re: Battery questions

that does look concerning. try swapping that battery into the middle of the bank. battery d to h and battery h to d. try to equalize them a bit more and see what happens with those readings, but before you do anything take the at rest voltages for the batteries individually. i guess i should ask if you have a dmm to use to take these readings? if not then just go ahead with the battery swap a eq and recheck your sg. i am assuming that you have been topping them off with distilled water only as regular water introduces impurities and have there been variances in the water added to these cells? if you have the dmm then check both the sg and voltages before and after you have done the battery swap and eq.
if you have the dmm then we may be able to do more tests concerning your wires and connections if this problem persists.

BB. · January 2009

Re: Battery questions

Since you have one series string--there should be no difference (in charging current) between the beginning and the end of a string.

Issues may include--differences in battery temperature by position (cooling air, bad connection or wiring heating up one cell or so). And/or you are not properly equalizing the batteries (all cells reach 2.xx volts for yy minutes, etc.).

Certainly, contamination of electrolyte (battery distributor, or you when checking cells/cleaning tops of batteries) or even a "bad battery" are all possible.

Use a good DVM (digital multimeter) and check the voltage drop across each connection/wire when the bank is under heavy load (or heavy charging). If you see one point to point drop that is significantly higher than any other--you may have a poor connection there.

How often do you need to add water... Adding some to every cell every month or so is normal. Not needing to add water at all (or one cell that takes more/less than the rest) is not right.

From the rest of your sg. readings--it seems you are getting the rest of the bank pretty fully charged. Can you measure the cell voltage for each battery (or are the cell interconnects not available).

If one battery/cell is higher voltage than the rest (when charging) or lower than the rest (when discharging)--you probably have a problem there.

-Bill

peterako · January 2009

Re: Battery questions

The low reading is maybe based on a lower acid level in this cell. it is clear not bad, but only a lower reading. i worked on submarines and it wash not possible to move cells around. there where spare cells but it wash around one day work to move out one bad cell, so relocating as maintenance is not required.

your equalizing for 3 to 4 hours is long and a wast of energy and battery liquid. one hour by 63 Volt is enough ( i am only doing this for 20 min. ones a month).
The meaning is to circulate the liquid in the cell by boiling and bring the cells on level.
I never have all the cells on one level.

Another point is a low temperature if there are cells close to a cold wall the chemical process is slowed down and so there is a lower capacity ( check the manufacture for its diagram from the capacity by temperature.

Greetings from Greece:cool:

peterako · January 2009

Re: Battery questions

And one point escaped me before.
if you use welding cables be aware the rubber is not acid proof so the acid mist is slowly going in the cable and are there eating your copper.

i advice to use only battery cable and seal or crease the cable connector.

BB. · January 2009

Re: Battery questions

peterako wrote: »

i advice to use only battery cable and seal or crease the cable connector.

A question--are you typing about "sealing" (with silicon seal, anti-corrosion paste, or soldering?) and "crease" as in "Crimp" the connector (to use a tool to "shrink/swag" the metal connector tightly to the cable itself).

No complaint here--You are doing a lot better in English than I could do in Greek (or probably English for that matter).

Any other information from you Submarine days would be very interesting too.

-Bill

niel · January 2009

Re: Battery questions

well he isn't in a submarine and i'm sure these batteries would be no match for the quality seen in a sub's batteries. batteries near the outer ends of a string do get hit with more abuse so i still would say move that battery into an interior position of the bank no matter what the initial source of the inequality may be, but he should track this with voltage readings for the batteries he intends to switch minimally and tracking all of them is the better idea. individual cell voltages can be done on 6v batteries, but i wouldn't normally say to do such a thing as this may risk adding impurities to the cells. just measure the voltages for each individual battery. it should be done with a somewhat good digital meter so as to see better accuracies.
bill did go further into what i mentioned in checking your wires/connections and you can still do this, but try the battery swap first as its battery position may be putting undue stress to that battery, if so and you swap them then you will see an improvement to that battery with the weak cell in time. as was mentioned by petereko, you could also reduce the eq time while still doing this monthly. 1hr should be ok or check with the battery manufacturer for recommendations on eqing.

icarus · January 2009

Re: Battery questions

Trkarl,

If the offending cell is proved to be bad, at least with the t-105's you could buy one new one and add it to the string. Probably cheaper than letting the bad one bring down the entire string.

Depending on the age,, adding a new one isn't the best idea, but none the less...

Tony

trkarl · January 2009

Re: Battery questions

Thanks for the responses. I did move the battery in place of one of the middle ones. As far as the eq time the Trojan website says:

"Equalization is complete when specific gravity values no longer rise during the gassing stage."

I generally check it every half hour on two or three cells until it no longer rises. It usually takes between 3 to 4 hours for that to happen.

The welding cable is excelene brand which is supposed to be acid resistant. I have it on a 12v system that is 5 yrs old and I replaced the lugs for the inverter cables yesterday which were badly corroded. I cut the old ones off and the copper inside was fine so I don't think it goes through the rubber, at least it hadn't in the past 5 years on those cables.

Also I top them off every 4 or 5 weeks with distilled water.

We'll see what happens.

Tim K

mike95490 · February 2009

Re: Battery questions

Batteries in a series string, they ALL see the same currents. Position means nothing, other than temperature, if the center ones are heated by neighbors.

I think crease meant grease, like vasoline, to seal the terminals from acid vapor.

Try this, find your low cell, and remove it from the pack, and give it it's own equalize charge for a couple of hours, then return it to the pack. Might save you from boiling the whole bank for hours, just to top off one cell.

BB. · February 2009

Re: Battery questions

trkarl wrote: »

I have a desulphator permanently attached to the bank. All the interconnects are the same length.

By the way, check how much current the desulphator is taking... Personally, I have not seen anything that shows a desulphator preventing/fixing sulfation problems. They are just another load on the battery bank wasting energy (in my humble opinion).

Cable lengths should not be an issue in a series connected battery (short, heavy gauge cables always being a good thing).

Taking a DVM set to 100 or 200 mV full scale and checking every cable/connection junction point (i.e., battery post to terminal lug, terminal lug to cable, left end of cable to right end of cable, rt cable to cable lug, lug to battery post, etc.) -- all done while the battery bank is under heavy load (or heavy charging). And connection with too much voltage drop (or even "too little" voltage drop) can indicate problems with a connection (or no current flow--common issue with parallel strings of batteries--one string carries most of the load current, the other near zero because of corrosion under a lug, etc.).

Certainly, you can do a quick check (battery lug to next battery lug DVM voltage drop reading)--and if you find an issue on the overall drop--then measure each accessible junction point to locate the exact problem.

-Bill

niel · February 2009

Re: Battery questions

mike90045 wrote: »

Batteries in a series string, they ALL see the same currents. Position means nothing, other than temperature, if the center ones are heated by neighbors.

mike,
although you are correct on equal currents, it has been known that failures usually occur on the outermost batteries and not just for lead acid types either. if it were a heating issue then the innermost batteries would experience the failure and not the outer as is the case with trkarl's outer battery failure. rearrangement of the batteries is helpful in eq and by your way of thinking there would never be a need for an eq in a series battery bank and that isn't the case.

mike95490 · February 2009

Re: Battery questions

I never said that series strings would never need EQ! Only that series strings all see the same Current (Amps) You can have a battery still die anywhere in the string, but since in Series, all current is the same, except for thermal issues, I don't see how shuffling batteries could affect them. 10 amps at one end of the string = 10A at the other end.

This still allows a cell to go bad, and need EQ, and that would work as always, boil the other cells till the low one starts to boil too!

Maybe the outer batteries see more stress in cable flexing/terminal torque or something, but there is nothing electrical to induce outer batteries any differently than inner ones.

If you have parallel strings, that's a whole different ballgame.

peterako · February 2009

Re: Battery questions

I agree with mike.
Before replacing it is maybe a trick to replace all the liquid in the cell for fresh acid/water mixture. i have seen it as a maintenance every 5 years by a telephone company.

;)And if you see any typing mistakes it is my dyslexia not my Greek ( i am dutch living in Greece )8)

trkarl · February 2009

Re: Battery questions

After moving the battery toward the center of the string I ran the batteries to about 60% dod and then charged them. Absorption time was about 6 hrs. at 61.4 volts. Checking the sg the bad cell then read 1.195 while the cell next to it read 1.280 . I am using this as an excuse to order the surrette batteries and replace the string with the 6cs17ps 6 volt surrettes. This brings up another question.

In the battery comparison chart in the Home Power 127 issue it shows the surrettes as rated at 5000 cycles 20% dod and 3300 cycles 50% dod.

If say I cycle it 20% dod normally per day would it be more advantageous to charge them every other day at 40% dod and theoretically get 6600 days of life instead of charging every day and get 5000 days of life?

I understand that age, temp, etc. also affects battery life so the numbers would not work out exactly but would I gain a life cycle advantage? Even an extra 500 to 600 days of life would be about an extra 1 1/2 years of life.

Tim K

BB. · February 2009

Re: Battery questions

There is a recent (long) thread asking pretty much the same question:

Time to Question the 3 day Rule ?

If you have a choice to charge with a generator after 1 day or 2 days (for example, you know it is going to be cloudy the next day)... (also, similar question is it better to have a huge bank cycling depth of discharge of 10% or a smaller bank cycling depth of charge down to 40%).

My two cents, it is not an obvious answer... Batteries that cycle deeper do not last as long. However, when you look at the depth of cycle versus battery life--you find that there is not a huge difference... If you charge every day (fixed, said every "3 days"), the battery will last 3-4x longer (in terms of cycles). But if you charge every 3rd or 4th day, there are 1/3 to 1/4 the number of charging cycles--very close to the same overall life.

And if you buy 4x the amount of batteries (1/4 the depth of discharge)--you could end up after a dozen years with almost the same cost... Do you replace 1 battery every 3 years, or 4 batteries 12-15 years... Not a obvious difference (buying more batteries up front--higher up front costs; battery costs go up over time vs inflation, etc.).

The big things to help batteries last longer... Charge them within 1 day after they go below ~75% state of charge (prevent/reduce amount of sulfate hardening). And keep the batteries filled with distilled water.

-Bill

niel · February 2009

Re: Battery questions

trkarl,
be careful as you read dod figures. dod or depth of discharge refers to the amount of power withdrawn from a battery put into a percentage. you may be reading that wrongly by mixing that up with soc, but anyhow the more you draw the lower the number of cycles. that does draw up a question in my mind though as to how they define a cycle. technically a battery left alone with no loads or charging sources has 0 cycles and would not be expected to last. we can all agree upon that one, but that same battery with a continuous float also has the same number of cycles as no full bulk charges or loads were presented and it would be fine after a year. in my mind that must refer to a specific number of ah in and out (can be speced as a percentage of capacity) and i don't see any of the battery manufacturers that i recall specifying that.

icarus · February 2009

Re: Battery questions

BB. wrote: »

There is a recent (long) thread asking pretty much the same question:

Time to Question the 3 day Rule ?

If you have a choice to charge with a generator after 1 day or 2 days (for example, you know it is going to be cloudy the next day)... (also, similar question is it better to have a huge bank cycling depth of discharge of 10% or a smaller bank cycling depth of charge down to 40%.

My two cents, it is not an obvious answer... Batteries that cycle deeper do not last as long. However, when you look at the depth of cycle versus battery life--you find that there is not a huge difference... If you charge every three days, the battery will last 3-4x longer (in terms of cycles). But if you charge every 3rd or 4th day, there are 1/3 to 1/4 the number of charging cycles--very close to the same overall life.

And if you buy 4x the amount of batteries (1/4 the depth of discharge)--you could end up after a dozen years with almost the same cost... Do you replace 1 battery every 3 years, or 4 batteries 12-15 years... Not a obvious difference (buying more batteries up front--higher up front costs; battery costs go up over time vs inflation, etc.).

The big things to help batteries last longer... Charge them within 1 day after they go below ~75% state of charge (prevent/reduce amount of sulfate hardening). And keep water with distilled water.

-Bill

This is the exact reason I went with T105s for my recent set as opposed to the L16's I used before. I don't remember the exact numbers, but by my calculation I could draw down the t105s lower, giving them a shorter lifespan vs the l-16s, since the t-105s fit the bill for load, and they were substantially less expensive, the net/net cost, over the life of the batteries is (I hope) going to be less with the t-105s, even though I will have to replace them sooner. This assumes of course that the batteries are loaded properly.

I used to run 4 l-16's that I bought used and they were fine, except I had trouble keeping them 100% charged. I also have my original t-105's that are still on a seasonal building, charged with one small ~60 watt panel with a PWM controller. These are now ~12 years old and show no sign of dying.

Tony

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