More battery voltage charge/discharge curves

[frackers]

I'm learning all about my new battery bank now as I play about charging and discharging it. One of the things I've learned raises an interesting issue with inverters but more of that later.

I started with the dump controller set for 24.5/28 volts for the limits and after a week the batteries were still well up and the discharge cycle was only running for about 20 minutes. Since I'm expecting about 2 hours (40 amp load on 450amp/hrs should give about 2hrs without going below 70% of full charge) I knew something wasn't quite right!!

I increased the upper limit to 29.5 volts and reduced the lower limit to 23.5 volts and as luck would have it we had a good solid 30km/hr wind for several hours to really top off the batteries. This is where I hit the problem alluded to above. The controller got above its limit of 29.5 but by this time the battery volts were swinging up to over 30 volts (due to the time constant in the controller) so when the inverter was told to switch on it refused due to an over-volt condition - kind of makes it tricky to do an equalisation charge under these circumstances!!

So I unplugged the controller from the inverter control input, switched off the turbine for a few minutes and once the volts had dropped off a bit I ran the inverter at full (40amp) load for 2 hours by which time the voltage had dropped to 23.9volts and the SG had dropped to where I'd expect to see it at the 60-80% charge level (difficult to get a very accurate reading without a lot of acid stirring!!)

Time for another twiddle of the presets in the 555 controller so the upper limit is now 28.5 and the lower is 23.9.

More wind forecast for the next 2 days so I should get lots more testing done!! The moral to this tale I guess is to know your batteries, not some hypothetical set :-)

[wooferhound]

What Type of Batteries do you have ?

[frackers]

8 off 225ahr 6volt - similar to a Trojan T105 but a 'home brew' New Zealand make called 'Thor'.

They look a bit like this (he says hoping the image isn't too large!!)

[SparWeb]

I think I missed the type of controller you are using.  Is it a homebrew/Ghurd design, or a commercial type?  If you have a temperature sensor adjusting battery voltage, then this problem may only get worse as it gets colder.

I can only speak from my experience with a Xantrex C40 and a cheap mod-sine inverter, but on cold winter days (the battery bank is in an unheated shed) the battery bank voltage would rise above 15V and the inverter would kick out.

I think your upper limit was too high, even for flooded batteries.  Even 28.5 sounds pretty high, but it may not be harmful, depending on the battery.  As you say, know your own battery set - start with the manufacturer's data sheets.  If you don't have them, then start out conservatively, and test from time to time to see whether your batteries are holding the charge you expect out of them.

Speaking of tests, could you give some more details about what you did during discharge tests?

[frackers]

The controller is the one based on the 555 timer chip that can be found here on this site

I've now run a complete cycle of discharge, charge and discharge again, the latter two under the auspices of the controller. The discharge time was about 1.5 - 2 hours (not quite sure as I have to look out of the window to see if the 1kw halogen heater is on!!). Overall I think this leaves my batteries safe until I build the real controller.

[SparWeb]

I experimented with the Compdoc circuits, too, until I realized that his tachometer circuit, designed for a 12 pole alternator, wasn't working so well on a 4-pole motor conversion.  The messy waveform basically precludes me from getting RPM info just by tapping the AC leads.

Anyway, back to the topic at hand...

Assuming the inverter is actually drawing 1kW, then maybe the discharge test is telling you that you aren't getting 450 Ah out of the bank, but proving that is a whole different matter.  Discharging down to 21V, at the rate specified by the manufacturer for load tests, may be a better way to demonstrate the practical capacity of your bank.  Without a data sheet, though, all you have to go by is the generic data that I gave you before.  Bear in mind that the capacity of the bank - the "450" number that you have, only applies to a specific rate of discharge, probably the 1/20 rate since they are knock-offs.  You are using a 1/12 rate, so naturally it will pull the voltage lower, sooner, and the capacity won't be as great anyway.  Your battery bank may be within spec, even though your early test results don't seem to show it at first glance.

One or two deep cycles won't kill the battery bank - provided that you don't do it often, don't go too far down, and that you actually use the resulting knowledge to take better care of the batteries, by avoiding excessively discharging them for the rest of their life cycles.  On the other hand, deep-discharges on new batteries aren't necessary unless you have reason to think there's a problem.  All the rules and recommendations for good battery care will probalby do you fine.

It's still fun to try, but do you have a "Peukert" chart yet to make sense of the data?