Battery voltage charge/discharge curves

[frackers]

I'm expecting new batteries today so I want to look after them. To this end I want to make sure that I don't over charge or discharge them. My controller is presently the simple 555 comparator circuit which I believe will do the job provided I can work out what assumptions are made when lead acid cell charge/discharge curves are published.

For example, I've seen many tables/graphs of cell voltage against state of charge but these are (presumably) plotted by charging, resting and then measuring voltage. How does this change if I'm measuring voltage whilst charging. The same goes for discharge.

The new batteries are 2 strings of 4 T105 equivalents for a 24volt system so I was going to start out with a full charge voltage of 28 volts and max discharge at 22 volts - the idea is to keep in the top 30% of full charge. Do these values look reasonable or should I start out with values closer to the nominal 25.2 volts (i.e. 2.1 volts per cell).

I could be living in interesting times!!

[Flux]

Unless you have significant periods without charge or load then volts will give you little indication of state of charge.

The values during charging and discharging are very current dependent and nothing like the values quoted when standing with surface charge removed.

Specific gravity is a far better indication for wet cells.

Flux

[DamonHD]

Here's one table...

http://solarjohn.blogspot.com/2007/03/measuring-battery-state-of-charge.html

Rgds

Damon

[frackers]

The batteries have arrived as promised - the supplier took the half dozen recycled truck batteries that I'd finished off in exchange for the delivery charge - he's happy and so am I :-)

They are all sitting, fully charged, at 6.39 to 6.41 volts so I'm going to set my dump load voltage to 6.75 volts per battery (27 volts rather than the 28 I was going to use). On the discharge side of the equation, I'm again going to err on the side of caution and cut out at 23 volts. I'll keep an eye on the specific gravity over a few days/weeks and see what happens.

Thanks for confirming my suspicion that I can't really use voltage, especially if charging at a high current. Guess I'll have to build the proper charge monitor/load-controller by counting amp-hours in and out of the battery bank. Got most of the bits, I've just got to clear the decks of all the other projects which may take a while with spring just round the corner!!

Cheers

[frackers]

Thats a darn good read - I'm revising my last revision of cut in/out  voltage to 28/24.5 - my previous ideas would have discharged the batteries way too far.

I wonder if that is what happened to the truck batteries...

[Flux]

I think 27v will do as float but you will need to equalise regularly if you keep it that low. Keep an idea on the sg. ( I would give them a good equalising charge now and check the sg at the end for future reference)

The table that Damon gave is about as useful as I have seen, but you really need to have figures related to charge and discharge current for your capacity of battery. The voltages change a lot and "significant current" is pretty vague.

The Ah meter is the best solution but even that is far from easy unless your charge and discharge currents don't vary much. If they do then you need Peukert as well as a general figure for efficiency. Again the big issue is a reference and the only thing you can reasonably use is full charge ( after equalising probably). If you can get an Ah meter to track sg you have about cracked it.

Even sg is not foolproof unless you gas enough occasionally to stir the electrolyte and avoid stratification. If you just use 27v without gassing it will eventually stratify.

Flux

[wpowokal]

All of the above and, follow the maker's recomendation, most if not all reptuable makers post their battery data.

allan down under

[kittysmitty]

Help me understand the effect of tempature. Acording to the chart a fully charged 12 volt battery is at 12.7 volts at 70 F plus or minus 5 F. If the temperature is 35 F, what would the voltage be, with a fully charged battery? Thanks

[wpowokal]

This will help

http://www.trojan-battery.com/BatteryMaintenance/Charging.aspx

Allan down under

[SparWeb]

Hi Frackers,

I've been searching on and off for days trying to locate a link on the web to a very valuable article that I found years ago.  It has a very good write-up and several very useful charts that relate the voltage, state of charge, and the current being charged or discharged from the battery at any given time.  I have it printed out (somewhere - last night I couldn't find the paper copy except the last page) but it is absolutely not turning up on Google.  I also didn't save a PDF, either, but if it interests you, I could scan it (5 or 6 pages).  I'm sure I can find it under my paper mountains if I make a dedicated search.  It must have come from Homepower (1990 or so), but I can't find it in their archive either, of course.

I have looked at the other links that members have provided to you so far in this thread, and that info is helpful, but this article beats them, hands down, IMHO.

[frackers]

Thanks for the offer but I've been told we have a copy of "Handbook Of Batteries" by David Linden and Thomas B. Reddy at work, thoughtfully pointed out by the guy in the next cell to me who designs battery chargers for a living!!

Guess what I'll be reading (all the pages relevant to lead-acid out of 1200 anyway) for the next few weeks:-)

[SparWeb]

Found it!!

http://www.otherpower.com/images/scimages/4378/Battery_Charging_HomePowerMag.pdf

Hey if you really want to go DEEP into lead-acid battery design, I've kept a whole bunch of that, too, along the way.  Most of it is for the commercial or university audience, so the short and sweet approach from HomePower suits me better.

Good reading!

[frackers]

Now that is exactly the sort of curves I was expecting and quantifies them so that I can see what happens at my normal discharge rate of C/10 (I only have the one load which is fixed) and (of course with wind power) charging from 0 to C/10.

I'll have to compromise on the charge side to C/20 rate so that I don't overcharge if I happen to have winds generating less than C/20 but since this voltage corresponds to a C/10 rate of 90% max charge at worse I'll simply have a short discharge cycle in a gale!!

Much happier now that I'm not going to dry out or sulphate the batteries.

[tsakach]

Here are graphs of actual real-time measurements from my solar system:

The measurement system records volts, amps, watts, amp hours, kilowatt hours and battery temperatures. Samples are recorded at 1 minute intervals, and detailed measurement data is stored of time frames for 24 hours, 7 day and 30 day periods. The solar system is powering a wireless community network hub and draws a constant load of about 45 watts during a 24 hour period.

In the volts graph, charge cycles are shown for discharge, bulk, absorb and float during a 24 hour cycle. The graph shows the MX-60 charger made an adjustment to the float voltage due to an increase of temperature.

The amp graph shows the characteristics of the current during the charge cycles, including variations in charging current due to cloud cover. A typical curve for absorb current is shown the graph, where current is reduced while the charger maintains a constant voltage.

The amp-hour and kilowatt-hour curves reveal some interesting information about the relationship between battery charging efficiency and kilowatt-hours during charge and discharge cycles. During an equalization charge or colder temperatures, the absorb voltage of the mx-60 will be higher. Higher voltages increase the ratio of charge/discharge hours, indicating more energy is dissipated to reach a full state of charge.

Finding the proper voltage based on battery temperature and battery technology is essential to avoid overcharging the batteries and optimizing system performance. This measurement tool provides a high level of visibility into the system performance, and I have found it to be extremely helpful in understanding how to obtain the best performance from my system.

[TomW]

tsakach;

Can you elaborate on how you are gathering the logging data and what you use to make the graphs?

I use a mix of RS232 meters, the Outback Mate, a linksys "Slug" and Linux / perl / sed / awk to massage the data from flat files and gnuplot to do graphs all on the Slug running Debian "Etch".

Mine are all offline now during equipment upgrades but thats how I do it.

Thanks.

Tom

[tsakach]

Tom,

Here are more details of the system:

This is new product based on industrial measurement and control technology, but designed specifically for use with alternative energy systems. There are two platforms supported, embedded Linux and personal computers running Windows or Linux. The data is gathered from a microcontroller module equipped with sensors for current shunt monitoring and temperature measurement.

The graphs shown are generated with the embedded Linux system, which in this case is a Linksys router running Openwrt and equipped with the microcontroller module. The router is handling logging and graph generation through a combination of RRDTool, bash, awk scripts and c-language drivers for device control.

The microcontroller handles measurement and control functions, and in some respects is similar in function to the Tri-Metric meter, but with more advanced capabilities .

In the PC platform version, the microcontroller includes on-board logging to capture data when the PC is not connected. The PC version has an Adobe flash web interface for device configuration, real time monitoring and graph generation. I will skip details on the control portion of the system, but the purpose is for adaptive load control to take advantage of periods of excess energy production, and for controlling stages of power-down during battery discharge.

The product is competitively priced, somewhere between the Tri-Metric and Outback Mate.

At this time, the product is still in beta testing. If anybody here is interested in participating in beta testing, let me know - a generous discount is available for beta testers.

Tim

[TomW]

Tim;

I am always interested in new tech. Linux is my preferred OS and the Linksys NSLU2 is already set up with Linux.

I will get an email off to you in the next day or 2 to get more information privately.

Thanks.

Tom

[DamonHD]

Always interested, but I (a) have a very small off-grid system and (b) am in the UK, so do either of those prove fatal to beta testing?

Rgds

Damon

[tsakach]

Damon,

Thanks for the interest in beta-testing. Your small off-grid system and location should not be a problem at all. I will send more information to your email address.

Regards,

Tim