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Batteries or How I let the deal go down !!

Posted: Wed Aug 26, 2009 7:10 am
by Jerry Campbell
I have been using my electrical system a lot more than normal this summer. I’ve been sanding and painting my bus. While using the pressure washer I heard the inverter beeping so when I finished the job I went inside to see what was going on and I smelled Battery. The inverter said low voltage. The voltage was 11.9 volts. Normally I wouldn’t check the water in them for at least another month. I have 8 Trojan L-16H 6 volt batteries hooked together as 4 sets of 2 for 12 volts. One was completely dry, one had two dry cells and one had 1 dry cell. I filled them with water and have charged them for three days. Yesterday they reached 13.3 volts, normally it goes to 14.8, this morning they were back to 11.9.
I broke my hydrometer not too long ago, so while I’m waiting for another to arrive I am wondering this, When a good battery goes completely dry could you or should you add electrolyte to try to bring it back or as in my past experience is it just dead.
BAD ME :cry:

Posted: Wed Aug 26, 2009 8:35 am
by Sharkey
Yep, for sure, bad you!

The bad news is that battery plates that have been exposed to air and dried out are lost forever. If the electrolyte was just barely exposing the plates, it might be possible to recover the cell, but you should only add as much distilled water as necessary to cover the plates before completely recharging. After the battery is fully charged, you then top up with water to the full mark.

How much charging current were you hitting them with? Eight batteries in that configuration would be over 1,260 ampere-hours. You'd have to be charging them at a rate of over 20 amperes, 24 hours a day to completely fill them in three days. For best battery health, your charger should be capable of hammering them with 120 amperes at the beginning of the charge.

Here's what I recommend:

IMPORTANT!!! Take a felt pen or grease pencil and number the batteries so you can identify them as you work on them. I usually keep a written log describing the battery before, during, and after testing. Make sure you keep track of which batteries had low cells.

Disconnect all of the batteries. Let them sit for 24 hours. Use a digital volt meter to check battery voltage. Checking individual cells would be better, but modern battery design doesn't permit this.

DO NOT add any water to the batteries except as described above to cover any exposed plates.

Pair up the batteries by open circuit voltage, and connect them as a 12 volt pair. Try to keep the battery voltages as close as possible to their neighbor.

Connect the charger to a single pair and charge until full. Don't stop until the voltage is up and you see lots and lots of very small bubbles rising through the electrolyte.

Set your charger (are you using your inverter?) for an equalizing charge. This normally means 2-4 hours or more at an elevated voltage, usually 15+ volts. This should cause the batteries to really gas hard.

Disconnect that pair of batteries and do the same to the next pair.

Repeat until you have gone through all four pairs.

At this point, you should be able to identify terminally dead cells. Look for batteries that have cells that don't react to charging in the same way, that is a battery with lots of gassing in just one cell, or a cell that refuses to gas when the rest on the battery are bubbling away.

Once you have done this, you'll want to repeat the battery pairing by open voltage again, discarding any batteries that have cells that don't seem to be charging properly.

MOST IMPORTANT: DO NOT DELAY!!! Disconnect those batteries this very minute. The longer your batteries sit in a bank with bad cells, the more damage you are doing to them! Cells that have shorted will consume the charge from the rest of the cells in the pack (all of them), and cells that have failed open circuit will prevent the rest of the cells in that battery string from charging completely, if at all. Rapid action is required if you intend to salvage any of your batteries.

I'm concerned that only a few of your cells had low water. This indicates that the cells may have been bad for a while. Usually, batteries with matched cells use water at pretty much the same rate, assuming that the batteries are all exposed to the same operating conditions (charge, discharge, temperature, etc). There are methods of connecting battery series/parallel strings that are more beneficial than others, where and how you make the connections can make a big difference in battery life.

Good luck with it. Dead lead sucks, and is a weight wround your neck.

Posted: Wed Aug 26, 2009 2:03 pm
by Jerry Campbell
I disconnected the batteries and all measured a little over 6 volts except one, it was 4.3, so at least one is dead. I'm going to mask and prime tomorrow so I'll let them sit until thursday check them again and start the pairing and charging process to see how many I'll have to replace.
I WILL be getting an automatic watering system.
Thanks again for the advice.

Posted: Sat Sep 26, 2009 10:30 pm
by AccordGuy
Bad luck Jerry,

Like Sharkey said, it's very important that the bank is wired up so that equal current (charge and discharge) goes through each battery or else they'll get more and more out of balance over time.

With two packs in parallel, it's easy enough to arrange as you just have to connect the load in a diagonal pattern so that the load is connected to the the (+) of one battery but the (-) of the other. That is, not the poles of the same battery in the parallel pair. If the interconnects are all the same wire size and length then it should keep in balance.

More than two in parallel and it's usually best to tie all the string (+) terminals to a common point and the same with the (-) end. It means using more wire but then the batteries can be (within reason) as far apart as you need (unavoidable on a big bank). DO connect them to a SINGLE point if you can. If you can't and have to use a bus bar to common the terminals, factor the length of the bus bar into your wiring lengths. So assuming you connect the load to the middle of the bus bar and the batts to points left and right of the load point, factor in that the two battery terminals at the far right and left ends of the bar have an additional few cm of "wire" in the length from the load point to the actual battery pole. The bar may be thicker than the wire you're using (hopefully it is) but you have to remember that the bit of the bar that is between the post on the end and the middle is carrying the sum of all the currents fed into it. So the last 1-2 cm of bar may be carrying a large multiple of the individual current of each feed wire.

So, for the terminals at the end of the bar, make the wire to the battery post a bit shorter than the ones closer to the load point. How much shorter to make them is dependent on their resistance per metre and that of the bar.

Even if your wiring is perfect, you can still get into trouble with a weak battery that has a tiny bit higher internal resistance than the ones in the other parallel paths and the current will bias towards the good ones (over charging them) and away from the bad one (undercharging it).

Using a bus bar might actually help here as you can tune the resistances of the interconnects a bit by moving battery terminals from one post to another to deliberately compensate for different battery internal resistances. You can make a spot check now and then by applying a large load or charge current and measure the current in each leg by using a mV meter to measure the voltage drop on equal lengths of the feed wires in each leg. You want to keep them identical (as much as possible). For 35mmsq wire, 18cm roughly equals 0.1mV per Ampere flowing in it (at 25'C). If you make some taps on the wires for measurement (I stuck brass pins into the wires and then soldered on test probe wire for my digital voltmeter) you can easily measure the current in each wire without the trouble and expense of cutting in a "proper" meter shunt.

It's all this nonsense that is leading me towards using fewer more massive batteries and higher voltage inverters. I did have my four 12V batteries in parallel feeding a 12V 1kW inverter but I wired it badly at first (the classic all in a line bus with the load at one end mistake) and they got horribly out of balance.

BAD - all in a line bus with the load connected somewhere in the middle to one battery.

Now I have a 24V inverter and only two strings in parallel (so I can use the simple diagonal wiring pattern).

GOOD - Each parallel pair connected in diagonal (so top pair are connected at top right and bottom left) and then in series so all the parallel battery interconnects are the same length. Load connects to entire pack at top right and bottom left.

I'm looking at getting more capacity (220Ah at 24V isn't enough). Rather than more 110Ah 12V batteries, I'm looking at just the same 4 battery arrangement but using 270Ah batteries or even 12x 580Ah 2V cells so that there are no parallel strings. I found an outfit that sells old utility batteries for peanuts (£35 for 580Ah 2V cells and £65 for 1000Ah 2V cells). I'd buy the 1000Ah cells but where am I going to put 1200kg of batteries...? And I've only got 1kW of solar to charge it with (oh and a 3A 24V charger I picked up at a car boot sale).