What is 50% discharge?

Freewilley

I see different charts and advice how to calculate when you have reached 50% discharge.

On another thread, which I do not want to divert with this question, I was told that 50% state of charge is 1.200 SG.

But on http://www.solar-electric.com/deep-cycle-battery-faq.html#Battery Voltages the chart says that 50% is 12.06 volts, which according to the Surette manual is an SG of 1.155.

Are we talking apples and oranges with discharge rates? I know that batterries can only be discharged to 50% if you want them to stay healthy. Is this the SG of 1.155? Are users then taking 1.200 as a better bottom cutoff because this level is in fact 1/2 of 1/2 of the usable capacity?

When I RVd with golf cart batteries, no solar, I thought I was advised that 10.8 was the real bottom and that it was ok to take them down to 11.9 as that was roughly 50% of capacity.

I have googled around this issue and am still confused by what I was just told.....

thanks!

NorthGuy

Re: What is 50% discharge?

There's no one-to-one relatonship between SG and SoC.

SG measures the proportion of acid in the electrolyte. When batteries are charged, you get a lot of acid. When they get discharged, the acid gets converted into water, which gets dissolved in the electrolyte and you see SG dropping. At this stage, you can use the charts to figure approximate SoC by measuring SG.

However, when you start charging, the water turns back into the acid, but acid doesn't dissolve in the electrolyte well. Rather, it forms big blobs that fall on the bottom. Since you do your SG measurements on the top of the can, they are affected by this process very little. SG increases very little during charging and you cannot use it to judge SoC.

When absorption starts, you get gassing and bubbling. This process rises acid from the bottom to the top and intermixes it with the water. At this point SG increases rather quickly until it reaches almost full state.

At the end of the absorption, there's almost no charging, most energy goes into mixing the electrolyte, but it still takes quite a bit of time to complete the mixing and elevate SG to the full level, at which time you know your batteries are charged. Thet's where the charging should stop. The art of battery management is to find settings which achieve this at least once a week.

Cariboocoot

Re: What is 50% discharge?

The problem is that none of it is linear and not all batteries are alike. So you can have generalized statements of SOC based on SG or resting Voltage, which is what we tend to use here, and specific recommendations from battery manufacturers. Erring on the side of caution results in longer battery life most of the time.

Repeatedly taking 12 Volt batteries down to 10.8 Volts, for example, is a sure-fire way to shave years off their lifespan.

My usual recommendation of setting LVD to system nominal (under load) sounds weird, but it ensures that the resting Voltage will be above a 50% SOC level no matter what batteries/system.

You can get into some pretty crazy debates about the economic feasibility of using deep cycle batteries to 'X' percent over 'Y' years resulting in a cost per Watt hour over lifespan equaling 'Z' - and the doggone batteries can then just quit at five years and ruined the plan.

Photowhit

Re: What is 50% discharge?

Cariboocoot wrote: »

Erring on the side of caution results in longer battery life most of the time....

Very true, and prudent. I'm not a big fan of those who are comfortable taking a system regularly down to 50% DOD, but then again I don't want to interact with my system every day. Some people want to, and that's fine too. I don't run a generator and don't plan on adding one to my system, which IMO would be required to regularly do these deeper discharges.

Cariboocoot wrote: »

My usual recommendation of setting LVD to system nominal (under load) sounds weird, but it ensures that the resting Voltage will be above a 50% SOC level no matter what batteries/system.

This I don't agree with unless your trying to stay in the upper 20% of your battery capacity, and even then you'll run into problems with lower voltage systems that run a microwave or some other high wattage short term loads. My guess is on my 4 golf cart batteries(cabin system soon to be someone else's), I can get very close to 24 volts just running the microwave for 3-5 minutes, even on a fully charged battery, with no charging.

After a while you'll get a feel for the SOC of your batteries. I posted a thread in OFF grid section about a high use day (Friday) and what my batteries 'looked' like in the morning. In it you can kinda see how I interrupted the voltage of the system, from the current voltage and the loads on the system.

Summer-going-out-with-a-bang-or-How-low-can-you-go

Freewilley

Re: What is 50% discharge?

But when we say "don't take your batts below 50% discharge or you are in trouble" and then you compare charts to find 50% discharge and it varies enormously, how do you determine the correct parameters.
A full battery is 12.7 say, and a FULLY discharged battery is 10.8 or less. 50% is often corelated to 11.9 volts, approx.
So I have been using 11.9 as the bottom cutoff for usability. This a.m. you warned me that I was below 1.200 SG and damage was being done. But 1.200 sg relates approx to 70% of capacity if I go to the Surrette chart on my wall, The chart DOES say that 11.89 is discharged, but I took that to mean they are calling a 50% discharged battery fully discharged because in practical terms it should not be further discharged.
This is very important to me, I am trying to understand these parameters and what I get is:
ring the alarms if there is 50% discharge of the total battery, which is at 11.9 or 1.150 sg. But when some people talk about a discharged battery they use that 11.9 parameter because in fact THAT is when the battery has no usable capacity left. Is it possible that other people see that 11.9 reading as a discharged battery and then back up to 50% of the difference between 12.7 and 11.9?

Vic

Re: What is 50% discharge?

Hi Willey,

From the Surrette Solar Battery Manual(in the past you DID say that you have a copy of this, and study it often), 50% SOC is 1.200 - 1.180, when compensated for electrolyte temperature. The Surrette Manual is THE place to look for information on how to charge and care for YOUR batteries. Some other batteries use different SG electrolyte to fill the batteries, depending the target use of the battery bank, and there are some different metallurgy involved in different battery designs that affect just how recharge must be done.

You have been working with Steve Higgins and Company at Surrette, so would first use the exact process that they have recommended for your exact situation.

Have you done a Commissioning Charge and & EQ for this battery bank? This has been recommended by Surrette, along with initial measurement of terminal voltage and SG of EACH battery/cell in a bank, and recording this data in your Battery Logbook. Have you done this?

In another Thread, you mentioned your SGs on this bank, which seem quite LOW. Know that you mentioned having a good Hydrometer. But something definitely seems wrong with your situation. Having several Hydrometers and seeing the difference between them can be telling at times.

Also, just to make sure, Draw and expel two or three samples of electrolyte BEFORE taking SG readings on each cell. Make certain that the float is not sticking to any part of outer tube (hope that you are using a Glass Hydrometer). And as you know Temperature Compensation can make a bit of a difference if the bank is too far from 77 degrees F. Plus/minus 5 degrees F makes little difference in SG accuracy. It is also a good idea to take two or three readings in a single cell, and see how closely they agree. RINSE the Hydrometer two or three times with Distilled Water after EACH measuring session - this is very important, as acid residue left in the Hydrometer becomes sticky in time, and causes air bubbles to adhere to the float and sides of the Hydro causing erroneous readings.

If your SGs are as low as you have been stating, there may be very heavy discharges on the bank, poor RE energy input, OR the Absorb times are far too short, and at too low an Absorb voltage.

Getting the ACTUAL SGs up to spec is very, very very important. Sooner for this is MUCH better than doing so later.

It is good that you are paying attention to these new batteries, thinking about things, reading and asking questions. You must get actual SGs up soon, and then you can fine-tune Absorption times and voltages after that.

Just my opinions, Good Luck, Vic

Cariboocoot

Re: What is 50% discharge?

Photowhit: with most (good) inverters when you program LVD at 'X' Volts they don't shut off at that level; the Voltage has to be below that for a certain amount of time before they go down. This avoids the problem of big loads coming on causing shut down when the batteries are already at a lower SOC.

Not all inverters will do this of course.

But <11 Volts is really bottom-of-the-barrel low.

Most of the time we balance a system to run an average DOD of 25% (which allows a margin of 1 day for bad weather before starting the gen). So that with the system nominal LVD and the 10% peak charge current in most cases will get you a functioning system. But everyone's application is slightly different and allowances have to be made for that too.

As I said we tend to advise in general terms here, because you have to be in the ballpark before you can play the game.

Also as I said and Vic reiterated the manufacturer should have info specific to the batteries chosen and that should be followed when available.

Not only that, but temperature does play a big role in both SG and Voltage readings as does the equipment used to measure either/both. So you can get close to recommended numbers, but you may have to adjust a bit from that to get what actually works for you.

Unfortunately it is not as simple as "a 12 Volt battery has an SG of 1.**** and Voltage of 1*.** when at **% SOC". If it were I wouldn't have any questions to answer at all.

vtmaps

Re: What is 50% discharge?

Vic made some good points about using hydrometers. They can be very misleading due to the stickiness issue. Most professionals around here use refractometers rather than hydrometers. I prefer refractometer to hydrometer, but I don't own a refractometer. Recently I purchased a Hydrovolt (a european hydrometer) from Midnite solar and I love it. Built in temp compensation, no sticky problems, very easy to read .005 increments, doesn't dribble acid out the tip.

--vtMaps

Freewilley

Re: What is 50% discharge?

Wow, so much good information.
1. I have 2 glass hydrometers, one is OTC 4619 Professional Battery Hydrometer with built in thermometer
2. I did not know about rinsing between tests, thanks.
3. I have been lucky to have Steve advise on the past bank, I have not talked with them about this one as I only started it 30 hours ago, I thought I was going to be free and clear.
4. Yes, I have the Surrette manual and the chart but I still am troubled that it seems to reflect usable capacity instead of total capacity, but I will ask Surrette to clarify.
5. I am quite sure my problem is not heavy draw...it is that despite being 50% discharged, the charging system says..Hey, all good here, lets float this system....so charging goes way down and the batts never come up. Once I turn to EQ, the juice flows and up comes the SG readings. Its now at 1.24 despite only getting 1.1 kwh of sun since I switched to EQ mode.
6. I guess I am doing an intial EQ! You made me go back to the manual....they made that recommendation very, very subtly 2 pages after they talk about initial charge. I missed it!

Vic

Re: What is 50% discharge?

Hi Willey,

Thanks for the reply. Thanks, too, for the signature line (somehow I thought you also have an FM CC ...).

Regarding #5 above, Then if the MX is the only Solar Charger, the Absorption time and perhaps Absorb voltage must not be correctly set.

And, an EQ on Surrette Flooded batteries, needs to be within the range listed in the Battery Manual -- about 15.5 V. This should be adjusted manually to compensate for actual battery temperature.

Since your SGs seem to be lagging, would suggest raising the Absorption voltage to around 14.8 volts, and increase the Absorption Minimum time to about 5 hours and the Max time to about 6 hours, for starters. As you know, the target SG for these batteries is close to 1.265, with temperature compensation. When you are able to fully recharge the bank daily, or every few days (depending on RE energy availability) then you can begin to reduce the Absorb voltage and the time to something more reasonable. High Abs voltage causes more water use, and some additional plate erosion and a bit higher battery temperature increase during Absorb, but these things are much better than never getting the battery bank fully recharged once per week or so.

Since your Hydrometers show the SGs rising when doing an EQ, the Hydrometer is less likely to be the issue.

Keep at it, and you will find the combination to time and voltage that works for YOUR situation.
Good Luck, and please keep in touch on how you are doing. Vic

Freewilley

Re: What is 50% discharge?

Vic
Yes I have an FM60. Sitting on a shelf. I intended to expand my array this summer, but it will be next summer after buying new batteries and wrestling with getting all working properly.

Back to the theme of this thread (but thanks for all the good advice on my battery problems)....

This discharge notion had me spinning. It is critical to know where this line is, not just for the bottom line but for many facets of battery charging such as when absorption starts or where the cherry zone is between 100 and 80 %.
I started the thread to see what the source document was for the preposition that Surrette batteries have an ENORMOUSLY different 50% discharge rate. I was very active in an RV forum and boondocked with double T105 Trojans for a few years (no solar) in the last decade. The agreed to line on that forum was that 11.9 was the 50% rate so you should always stay above that.
When I started with Surette's 5 years ago I saw the Surrette chart Vic referred to that correlated state of charge and SG and used it regularly for that purpose. But I always figured that Surette had tried to make things simple and designed that chart to keep people above that 11.9 rate...so they used 0% at 11.89 instead of 50% (which gives many newbies the impression that they have lots of reserve).
Last week I discovered that there was another group who views that chart as representing total capacity, not usable capacity (my term). So I surfed around and found that this forum's host says this about deep cycle batts: (link is in first post of this thread)
Here are no-load typical voltages vs state of charge

(figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%.

State of Charge 12 Volt battery Volts per Cell
100% 12.7 2.12
90% 12.5 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75

This seemed to confirm my thoughts that we were talking apples and oranges. After some replies on this thread, I contacted Surette to ask them where they thought 50% discharge was...and the answer I got was;
A 12 Volt battery bank at 50% resting measures 12volts.

So, unless somebody can point me to another source, I put forward that the Surette chart should be read as usable capacity....with the supposition that of course you should cycle your batts in a different range than 100 to 50% on a regular basis (no gonna put down that range, leave that for another day....)

happy to hear what you think about my nutty ideas....

Cariboocoot

Re: What is 50% discharge?

Willey, you are correct: it is the difference between total capacity and usable capacity and it confuses a lot of people.

In some cases the difference is due to the type (not brand) of battery. For instance forklift batteries can be discharged to 20% of their total capacity without harm.

Some people program their battery monitors for usable capacity ('X' percent of total depending on desired maximum DOD). This can be a good thing if the monitor may be observed by someone other than he who understands it.

It is a shame, but to date no "perfect" battery has been devised wherein 100% of the capacity is usable and replaceable (or even that you don't have to "put back" more energy than you can get out).

BB.

Re: What is 50% discharge?

Freewilley wrote: »

ENORMOUSLY different 50% discharge rate... 50% rate so you should always stay above that.

Just to be clear--I think you are talking about Capacity, not Rate...

Throwing in discharge (and charging) rates adds a whole 'nother level of complexity (the higher the discharge rate, the less apparent bank capacity--I.e., C/20 and C/10 are light to moderate discharge rates... C/5 to C/2.5 for flooded cell are relatively high discharge rates).

-Bill

Blackcherry04

Re: What is 50% discharge?

50 % will always be 50% of something. One must assume that a constant is the voltage of 12.6-12.7 is standard. That said Electrolyte sure isn't up until a few days ago I thought 1.260, 1.265, 1.280 & 1.285 was the range I would normally expect. I guy brought me a couple Industrial Batteries to EQ in my tank he was having problems with. I took the SG level and was scratching my head, The label on the side ( In small print ) clearly stated " Filled with 1.315 Electrolyte ". In this world you cannot assume anything.