View Full Version : Battery weight vs. Charge

Chris Tremblay
03-26-2002, 02:18 PM
I have a question concerning the weight of a battery vs. it's charge. I one of the batteries (2002) is fully charged will it be heavier than one with a depleted charge. I have heard a believe this to be a fact. In some cases the battery I heard could be pounds heavier or just ounces. If anyone knows i would appriciate if they would respond. Thank You:confused:

Al Skierkiewicz
03-26-2002, 11:01 PM
This is one of those myths that pass around from time to time. I think it began because lead/acid battery condition is monitored by "specific gravity" measurements. Batteries in industrial UPS power, cars and even those old deisel/electric submarines checked the batteries with this method. (The father of one of our former students actually performed this measurement on subs and told me of his daily tasks on board) This is a chemical measurement where a drop or two of battery acid is used from each cell. You'll have to look this up for a complete description.
What is real with our batteries is this... As the battery goes through repeated charge/discharge cycles, the acid (electrolyte) evaporates and vents out the top of each cell. After many cycles there is a noticeable difference in battery weight when compared to a new battery. However, when that much acid has evaporated there is less lead plates in contact with the acid and the power density and recharge time goes down. If you suspect a battery is beyond it's useful life and it is lighter than a known good battery, it's time to recycle.
Good Luck

03-26-2002, 11:41 PM
why not charge one battery and leave another uncharged, and take them to the scale and compare weights? just a thought ;)

Al Skierkiewicz
03-28-2002, 06:34 AM
For pure scientific investigation you would need to measure the same battery, charged and uncharged. We have found differences in batteries of as much as .2 pounds which I would quess are within production variations.
Good Luck

03-28-2002, 03:11 PM
There is in essence no change in weight as a battery discharges.

Batteries are electrochemical devices. A chemical reduction reaction occurs, which includes an electron exchange to convert one molecule of "stuff" in the battery into another. A battery is set up so that an electron has to travel from one plate through the wire to the other plate to become "available" to complete reduction of a molecule of the battery chemistry to a lower energy form.

The power derived in a battery is the total energy difference between the two molecular forms, times the number of molecules in the battery involved in the chemical reaction. (In other words, a bigger battery = more chemicals to react = more total power. <Add Tim Allen Grunts here...> ;) )

Since all of the atoms involved are still in the battery before and after this action (in this case, air is not significantly involved as would be in a gas engine) and only their combination has changed, there is NO change in weight as it discharges.

However, what Al said is correct in that as you recharge the battery, you'll typically "boil off" some of the liquid in the electrolyte, or form a gas such as Hydrogen as a byproduct, which does reduce the weight slightly with each recharge. However, this change is very small WRT the total weight of the battery. Hardly noticeable (unless your charger is broken and boils away ALL of the electrolyte, in which case you have OTHER problems! :).

For our purposes, since the weight while discharging remains the same, your robot will <i>not</i> become lighter as it runs out of power (like a car does as by using up gasoline and throwing the chemical byproducts away as exhaust).

I believe you'll find the differences in plastic molding or plate material between serial numbers of a battery will <i>far</i> outweigh <i>any</i> chemistry "vanishing" effects from multiple recharges over the entire useful life of the battery.

I hope that clears this up.

- Keith McClary, Advisor, Huron High Team 830 "Rat Pack"

Andy A.
03-28-2002, 05:46 PM
Ok, here's my question:

I work on a electric vehicle class at our high school (build a ev from scrap in one year, a lot like FIRST Acutally). We use 8 deep cycle 12 volt batterys. One of my jobs is to keep them topped off with plenty of water, often after it comes off a charger. I've pretty much got vented batterys down pat.

Now, where I get confused is on sealed batterys. I understand that the chemistry is pretty much the same and for various reasons the electrolyte is in a gel, giving the name 'gel cell'. Now, since they are unvented and supposedly unspillable, how do we boil off the fluid? Is there really some venting going on that just isn't obvious?

-Andy A.

03-28-2002, 08:06 PM
This is from a battery FAQ at: http://www.optimabatteries.com/faq.asp

15. What is a Gel Cell Battery?

A gel battery design is typically a modification of the standard lead acid automotive or marine battery. A gelling agent is added to the electrolyte to reduce movement inside the battery case.

Many gel batteries also use one way valves in place of open vents, this helps the normal internal gasses to recombine back into water in the battery, reducing gassing.

Now to answer your question about outgassing:

Normally, if you charge them at the specified rate, they should NOT outgas. The gasses in theory should be reabsorbed. However, they WILL outgas IF you charge them at too high of a rate. In that case, the one way valve will let the gas escape for safety, but if you are generating gas you're damaging the battery.

"Smart chargers" (like those supplied with the contest) are SUPPOSED to charge them at a low enough rate that will NOT cause them to outgas. (That's why they take so long to recharge.)
However, I have NOT personally run any tests on the supplied chargers, and as rookies we haven't experienced any battery failures to date so we have no data on their lifespan. I just have to assume FIRST insured the selected chargers are designed correctly for the supplied batteries. :)

BTW... If you look up the battery specs, you see you can VASTLY improve the total number of cycles on the batteries by keeping them fully charged and NOT deep cycling them. These are NOT NiCds. You shorten their lives by something like a factor of 10 or more by running them into the ground before recharging them. You should always recharge them fully after each use. So make it a habit that EVERY time you use a battery, even for a short test cycle, you put it back on the charger overnight afterwards.

The batteries also lose a small amount of charge per month to internal discharge resistance. Therefore, to keep them in best shape, you should try to stick each of your batteries on the smart charger overnight at least once a month to keep them "topped off". The charger shuts itself off when the battery is fully charged, so you won't harm them even if they seem fully charged when you put them on it (or if you forget and leave one on it for a week or two :) ).

We serial number our batteries, and keep a "battery log" posted on the wall in our storage room by the charging station so we know the charge history of each one, and keep one on the charger at all times in a rotating fashion.

- Keith McClary, Advisor, Huron High Team 830 "Rat Pack", Ann Arbor, MI

Al Skierkiewicz
03-28-2002, 09:55 PM
Now we're getting into some really heavy stuff, pun intended! There is a lot of marketing hype in what we are discussing here so let me add some to Keith's post and to answer some of the questions raised by Andy.
In the deep cycle batteries you are topping off with water, distilled water I hope, you are replacing the water that evaporates during the normal charge/discharge cycles all lead/acid batteries experience. The battery temp is raised during charge and discharge causing some water to be released in a gas form in the battery. During charging, as current passes through the electrolyte, some bubbles form, rise to the surface and carry small amounts of water vapor with them. In a normally vented battery this vapor leaves the battery. In a sealed or maintenance free or call it what you will battery, there is sufficient space above the electrolyte for the vapor to condense and return to the cell. In all sealed batteries there is some kind of vent arrangement to prevent the battery case from cracking open any time the internal pressure rises. If the pressure increases because the battery was left in the sun, or because the electrolyte has frozen, there is very little water vapor lost. If the pressure was caused by charging, some vapor will escape when the battery vents. Over time this lost vapor will reduce the weight of the battery.
There are vents in our batteries, they are just hidden. The gell additive just keeps the liguid in when you turn the battery on it's side. That is a big factor in using these batteries in our competition. You cant turn them over and spill the acid. It is not an overly strong solution of sulfuric acid but it can be bad in eyes and on clothes. (clothes will disolve where there was acid when they are washed.)
Finally, the 4 amp chargers are in the range specified by the battery maker as being the highest charge current. This rate takes into account the amount of internal pressure reguired to "pass gas", a sufficient margin to prevent internal damage due to metal distortion of the plates and a quick enough charge time to maek them attractive power sources for general use. These batteries see service in such applications as exit signs and safety (loss of power) lighting and ham radio operators find them useful for portable operation preferring to charge them with solar cells.
To make matters worse there is probably a failsafe vent that is designed to open wide but only once. This is for the user that may have a voltage regulator fail during a charge or for extreme conditions of discharge or temperature, and is simply there to prevent explosion. (shrapnel and acid, lead contamination all really bad stuff)
Hope all this helps, thanks Keith for the added info.
Good Luck All

Gui Cavalcanti
03-31-2002, 12:52 PM
Wait a minute... you're saying batteries like to be kept charged. Does this mean that we're potentially damaging our batteries when we shipped them with the robot from VCU? We sealed them March 9th... we open the crate April 25th... is something going to go wrong? Should we not have shipped them?

Al Skierkiewicz
03-31-2002, 09:46 PM
You are doing fine with the batteries you shipped. You will find that putting on the charger will show they are discharged just a little. Top them off before going to the field. In most cases the batteries will retain their charge for months and will not be damaged if they are stored partially charged. A few caveats though, don't store where temperatures can be extremely cold or extremely hot or in a very moist environment. Always protect the terminals from shorting or damage. We have batteries that are still powering our practice robots that are several years old.
Good Luck All

04-02-2002, 12:48 AM
Here's the data sheet on the Battery:

As you can see, here's the discharge percentages for an unconnected battery:
1 month = 97%
2 months = 91%
6 months = 85%
So as you can see, it is not fast, but it IS significant if you don't float charge them for a year or so...

Also, look at the Life Expectancy numbers:

If you totally discharge them (like many teams do) you'll only get 180 cycles out of them, but you if you only discharge them 30%, you get 1200 cycles!!! That's over 650% better life expectancy!

Therefore, the "shallower" you discharge them, the better their life expectancy. In fact, you can even float charge them forever with the smart charger and it won't hurt them.

That is why I recommend:
1) Never running a battery dead,
2) Always leave one float charging (your charger should be kept busy),
3) Rotate all team batteries at least monthly onto the charger to keep them topped off,
4) Immediately recharge any battery used for even simple robot tests, and
5) During heavy testing, if you have lots of batteries, rotate through your batteries frequently even within one day (esp. between major tests) so no one battery is pulled down very far, then recharge them all immediately.

- Keith

Al Skierkiewicz
04-02-2002, 08:28 AM
Most of what you say is true, but not to confuse anybody...There is a difference between leaving a battery connected to a "smart charger" and leaving a battery connected to a float charger. The difference here is the smart charger stops providing current when the battery is fully charged and won't turn back on unless you disconnect the battery or cycle the power. A float charger on the other hand supplies current (albeit low current) for ever. Any current that is not used to charge the cells is converted to heat in the battery. The self discharge current will not damage batteries and not keeping them topped off will not significantly reduce their service life.
I am making some assumptions here based on our team experience and that is 1. A team will participate in some post season activities, demos or competition that reguire charged batteries. 2. During season, batteries are used frequently for approx. 4-5 months. 3. Batteries are stored at normal temperatures (72 or so) and are not pysically abused. (dropped, stored without terminals protected or used to power equipment that operates at less than 8 volts.
So what are the tradeoffs? Use a float charger to keep batteries topped off will raise the battery temp, risk evaporation of the electrolyte, consume power, and may under ideal conditions extend battery life. Use a smart charger which stops charging and just keeps making heat at the heatsink and transformer, no battery damage, and about the same service life. Or store in good conditions, bringing up to charge when needed for demo or competition, no battery damage, no power consumed, no heat generated, and about the same service life as the other methods. You choose what is best suited to your needs.
As a side note, this same battery is used in emergency lighting in buildings when there is a power failure. By regulation they are left on float charge and may never get used if the power never fails. Battery life here is no higher than for our batteries and most prudent building engineers will exchange all batteries at three years intervals or less.
Good Luck All

04-10-2002, 10:20 AM
Originally posted by kmcclary

If you totally discharge them (like many teams do) you'll only get 180 cycles out of them, but you if you only discharge them 30%, you get 1200 cycles!!! That's over 650% better life expectancy!

Actually, you're getting twice the lifetime power out of the battery in the partial discharge scenario you described.

Since you are only using 30% of the power in the second situation, you have to multiply the 1200 cycles by 30%, which makes it the equivalent of 360 full cycles, or double the deep-cycle lifetime power, which is still substantial.

Two other obvious advantages of keeping the batteries charged are that they put out more power when they are fully charged, and you know you have charged batteries to use.