NiMh vs SLA (AGM's)

12 posts / 0 new
Last post
MB-1-E
MB-1-E's picture
Offline
Last seen: 11 years 1 week ago
Joined: Tuesday, May 8, 2007 - 18:31
Points: 385
NiMh vs SLA (AGM's)

Ok, so I'm good for possibly the next three years with my AGM's. I plan to keep on top of battery maintenance so I hope I'm being realistic.

When it's time to replace my batteries I hope to move to NiMh or even maybe something else, battery technology is moving along about as fast as my MB-1-E.

I use NiMh for anything that needs a rechargable battery in my house except for the laptop and the cell phone. They seem to do well and last quite a while and are relatively easy to recharge using my AccuManager Charger.

My question is related to Amp Hours.
I currently have a 24V-36Ah battery bank on the MB-1-E.
These are of the SLA type.
Do I need 36Ah of NiMh to get the same out of my battery bank? I think probably not.
The reason I have 36Ah with the AGM's is so I don't deplete them more than 50% (or less) so that they will last a while.
It's my understanding that the NiMh will run nearly to depletion without harm.
I'm guessing that I can use somewhere around 20Ah of NiMh to get the same range from them.
Does this sound about right?

What about charging?
I assume the charge time will be a bit longer for this size bank of NiMh.

When purchasing NiMh, what's the best way to get the most Ah/$ ?
I use CTA 12000mah D-size for my most used battery operated lights. They take a while to charge but boy do they last a long time. I'm very pleased with them for that application.
Purchasing 40 CTA 12000mah D-cells (this would give me 24Ah@24V) is pretty darn expensive. We're talking over $500 if my calculations are correct.

Just looking for some advise on cost effective alternatives to the AGM's for my next battery purchase.

Thanks for any light you can shed on this (pun intended). :)

Fechter
Fechter's picture
Offline
Last seen: 11 years 6 months ago
Joined: Friday, November 17, 2006 - 07:01
Points: 199
Re: NiMh vs SLA (AGM's)

Under high load, AGM's will deliver about half their 'rated capacity'. Possibly a bit more if they are high quality ones.
Nimh will deliver nearly the rated capacity, so to get the same range, you won't need as many amp-hours.
One problem with Nimh is they generally don't do well at extremely high discharge rates, and the voltage will sag more. Battery heating may become an issue also.

MB-1-E
MB-1-E's picture
Offline
Last seen: 11 years 1 week ago
Joined: Tuesday, May 8, 2007 - 18:31
Points: 385
Re: NiMh vs SLA (AGM's)

Thanks Fechter,

My hope is that in three years battery technology has advanced so that we can see more of the NiZn etc. The NiMh packs that I looked at (I believe they were 10 D-cell packs had a 20Ah@12V discharge rating. Does that change as you change the quantity of cells? Would a 20 D-cell pack deliver 40Ah@12V?
My system is 24V. I may decide to go to 36 but I'm not sure of how it would effect my 24V 650W Scott motor. I know they are tough motors, perhaps it would handle it fine, I just don't know.

What I'm getting at here is to up the voltage and reduce the Amperage draw on the batteries.
Does this sound like a possible solution to the battery overheating?

Guess I need to go back to some electrical/ electronic classes ... :)

If you could give me some perspective on the relationship of how system voltage effects the system amps that my help me here.

I read a post where you said that most motors can be over-volted and that it's the current that makes a motor hot.

Thanks for your insight!

Dave

MB-1-E
Electric - Bridgestone MB-1 Mountain Bike
Icon Photo of lighning striking Eiffel Tower Jun 3, 1902, taken by MG Loppe'

Dave B

MB-1-E
<a href="http://visforvoltage.org/book-page/996-mountain-bike-conversion-24v-3-4h... - Bridgestone MB-1 Mountain Bike</a>

chas_stevenson
chas_stevenson's picture
Offline
Last seen: 8 years 10 months ago
Joined: Wednesday, December 6, 2006 - 17:14
Points: 1309
Re: NiMh vs SLA (AGM's)

Dave,

I think your guesstimate is very close. I figure it should be 21.6AH if you use them to 80% and 19.8AH if you use them to 90%.

As far as Fechter's comment he is correct, however with more capability, more AH, the voltage sag should not be as bad as it would be for say a 12AH pack. So the way I see it you may have a higher sag than with the 30AH AGM batteries but with 20AH it should not be too bad. scratch_head_thinking.gif

Fechter this is the way I understand it, please correct my thinking if I am giving wrong information. Maybe I will learn something too.

Chas S.
My Bicycle Pages

MB-1-E
MB-1-E's picture
Offline
Last seen: 11 years 1 week ago
Joined: Tuesday, May 8, 2007 - 18:31
Points: 385
Re: NiMh vs SLA (AGM's)

Thanks Chas,

As for the "voltage sag", I'm not familiar with what that is, obviously it's a drop in voltage but is this apparent at the beginning of use, during the major time of use or right at the end of use?

I know that the AGM's tend to lose voltage at time goes on, a fresh charge will give me 13+ volts but shortly I'll be running at a lesser voltage and the decrease seems fairly linear if I don't take it too far. I assume there is a curve with the AGM's that drops fairly quickly at the beginning, levels off somewhat then declines fairly rapidly after about 50%. How does this compare with the NiMh?

As always, I really appreciate your input and parcipitation here. That goes for Fechter, Chuck, others and the V-team as well.

Dave

MB-1-E
Electric - Bridgestone MB-1 Mountain Bike
Icon Photo of lighning striking Eiffel Tower Jun 3, 1902, taken by MG Loppe'

Dave B

MB-1-E
<a href="http://visforvoltage.org/book-page/996-mountain-bike-conversion-24v-3-4h... - Bridgestone MB-1 Mountain Bike</a>

Fechter
Fechter's picture
Offline
Last seen: 11 years 6 months ago
Joined: Friday, November 17, 2006 - 07:01
Points: 199
Re: NiMh vs SLA (AGM's)

The voltage sag occurs whenever the load is high due to the internal resistance of the cells.
If you put two strings together, the sag will be about half, and the range will be double. The only downside to that is you can't charge the two strings in parallel. You must separate the strings for charging. You can charge with two chargers or take turns with a single charger. The strings are then put back in parallel during discharge.

Increasing the cell count (voltage) and reducing the current may allow you to use a single string and avoid the charging issues while still providing the power needed. Depending on controller limitations, this might be a good alternative.

MB-1-E
MB-1-E's picture
Offline
Last seen: 11 years 1 week ago
Joined: Tuesday, May 8, 2007 - 18:31
Points: 385
Re: NiMh vs SLA (AGM's)

Thanks Fechter, fortunately I have a couple or three years to go on my AGM's. I'll definately do a lot more research before purchasing any replacements.
Charging the NiMh strings sounds just the opposite of my charging my AGM's ... as you may recall, I run the AGM's in Parallel/ Series (24v) but charge them all in parallel (12v). Sounds like another use for the DPDT switch.

FYI, charging my current system like that is working out great. The only problem I've run into is to remember to switch back to 12v before hooking up the charger. I didn't once and got a fair sized spark when connecting the charger, fortunately the charger is well protected and no harm was done.
The Small DPDT switch does work great and saved me from having to purchase another (24v) charger.

Dave

MB-1-E
Electric - Bridgestone MB-1 Mountain Bike
Icon Photo of lighning striking Eiffel Tower Jun 3, 1902, taken by MG Loppe'

Dave B

MB-1-E
<a href="http://visforvoltage.org/book-page/996-mountain-bike-conversion-24v-3-4h... - Bridgestone MB-1 Mountain Bike</a>

echuckj5
echuckj5's picture
Offline
Last seen: 10 years 7 months ago
Joined: Wednesday, November 29, 2006 - 07:13
Points: 394
Re: NiMh vs SLA (AGM's)

Voltage sag, battery pack size

lead batteries

Put a voltmeter on your car battery. Turn the starter on. Electrons drain off of the battery plate. The voltmeter is showing a voltage drop of 1 or 2 volts on a good battery sized for the application. In a static state, "open circuit" with no current flowing the potential difference, "voltage" between the poles of the battery is around 13 volts. Close the circuit and current "electrons" flow from one pole of the battery to the other and perform work, turning the starter. The voltage sag in this instance is caused by the inability of the chemical reaction inside the battery to re-supply the electrons to the battery pole at a rate fast enough to keep the voltage at 13 volts. This chemical reaction takes time. A larger battery will have less voltage drop, "sag" (keeping all of the other parameters the same)because there is more chemical to supply the reaction to keep the potential difference at the battery poles nearer to 13 volts.

The temperature of the battery has an affect on the chemical reaction in the battery. In the winter a car battery will have a larger voltage drop than in the summer because the chemical reaction in the battery is sensitive to temperature. The colder the acid is in the battery the chemical reaction in the battery to re-supply the electrons to the battery poles to maintain the potential difference between the battery poles slows down.

It has been 30 years since I studied batteries in my college physics courses. So I am forgetting some of the finer details, well really most of the finer details,but,, I am pretty sure this chemical reaction is exothermic, meaning it gives off heat which is wasted to heating the outside air. Now inside the battery a chemical reaction is taking place that is moving electrons which is generating heat. Another source of heat is being generated by internal resistance of the battery as electrons flow through the electrolyte and plates of the battery. As the battery warms up this internal resistance increases, non-linearly. This is a very non detailed explanation of the peukert effect.

Now, I hear that nickel and lithium batteries don't have a peukert effect so their internal resistance should be linear as it relates to temperature. Generally only the exothermic chemical reaction in these batteries is heating them on discharge. If the heat generated is not excesive there should be no harm to the battery structure (physically) nor to the electrolyte (for lack of a better word) and low internal resistance.

As batteries heat up, electrons being electrons, the electrons have a harder time flowing because internal resistance is increasing. Now, if these nickel and lithium based batteries heat up, there is an exothermic chemical reaction and an added increase in internal resistance heating the battery cells. Thus the need for parallel banks of batteries in lower volt packs and longer strings of batteries in higher voltage packs to help slow down the effect of the exothermic chemical reaction in the individual battery cells. Failure to size the battery pack to the application will overheat the batteries on discharge causing the battery structure and chemical (electrolyte) to break down.

chuck

[b]AGM BATTERIES[/b]

echuckj5
echuckj5's picture
Offline
Last seen: 10 years 7 months ago
Joined: Wednesday, November 29, 2006 - 07:13
Points: 394
Re: NiMh vs SLA (AGM's)

Now my question is
Well,,,,
How big should the pack be using nickel or lithium based batteries?

I don't think that the voltage system whether it is 24, 48 or 72 volts would be the issue.

Would not the Watt/hours of the pack be the determining factor?

If I am using a 1 hp 24 volt motor and some one else is using a 1 hp 48 volt motor and we both want to go 25 mph for 20 miles a trip would'nt our battery packs be the same size? The 24 volt system would have twice as many parallel packs as the 48 volt systems and the 24 volt pack strings would have 1/2 the batteries per parallel string. Overall pack size would be the same for both motors with the same average trip profiles.

chuck

[b]AGM BATTERIES[/b]

MB-1-E
MB-1-E's picture
Offline
Last seen: 11 years 1 week ago
Joined: Tuesday, May 8, 2007 - 18:31
Points: 385
Re: NiMh vs SLA (AGM's)

Chuck,

Thanks for the great explanation and questions!
Both were exactly what I needed. My last battery class was 37 years ago in the military and my plates needed de-sulphiting. :)

Dave

MB-1-E
Electric - Bridgestone MB-1 Mountain Bike
Icon Photo of lighning striking Eiffel Tower Jun 3, 1902, taken by MG Loppe'

Dave B

MB-1-E
<a href="http://visforvoltage.org/book-page/996-mountain-bike-conversion-24v-3-4h... - Bridgestone MB-1 Mountain Bike</a>

inventpeace
inventpeace's picture
Offline
Last seen: 7 years 7 months ago
Joined: Thursday, February 17, 2011 - 15:15
Points: 32
Re: NiMh vs SLA (AGM's)

Re: your comment: " I may decide to go to 36 but I'm not sure of how it would effect my 24V 650W Scott motor. "
DO NOT GO MORE THAN ABOUT 15% MORE ON THE VOLTAGE OR YOU WILL BURN OUT THE EXPENSIVE MOTOR. ie IF MOTOR IS RATED 72V DON'T APPLY MORE THAN ABOUT 83V OR YOU RISK DAMAGE TO MOTOR.
Also with higher amp (EV) type NIMH batteries REMEMBER there is a DANGER of melting and FIRE if you overcharge them, AND even more IMPORTANT: NIMH actually goes slightly DOWN in voltage as it nears full charge, this anomaly FOOLS most chargers and people, and results in melting and fire (one man rumored to have burned down his car and house because he did not heed this advice). The problem is that because of NIMH chemistry the voltage of each cell actually goes down (.1 v per cell?) which is hard to detect without special monitoring circuit, this one cell that is full charge then begins to HOG all the current and that creats heat which in turn melts and catches that cell on fire.
SOLUION: USE thermal overload relay to CUT OFF the power to charger when heat changes more than a small amount per minute or a set amount of overall temperature. Its not hard to make one using a thermostat which must be specially dialed relative to existing temp at start of each cycle (prius NIMH battery compuer monitors temp in about 6 places in each pack). ALSO you must monitor voltage as well to be safer and have a circuit (if you can program an EEPROM you can DIY) that will monitor each cell individually (prius computer has separate voltage monitoring leads to each and every of 28 cells), and cut of the juice when voltage after increasing for some time then decreases slightly ).. etc etc. BE CAREFUL WITH FULL SIZE HIGHER AMPHOURS NIMH>> they last forever if used between 40% and 80% according to toyota.
Stay away from "FULL RIVER" brand AGM lead acids. "ROYAL BATTERY" brands as they do not honor their 2 year warranties, also AGM's will pass load tests and appear to test well , but a constant load tester will reveal they are not good, they may give you only 50% of original distance after only 1 years light use.
Best Regards,

The Truth Sets You Free !

inventpeace
inventpeace's picture
Offline
Last seen: 7 years 7 months ago
Joined: Thursday, February 17, 2011 - 15:15
Points: 32
Re: NiMh vs SLA (AGM's)

ALSO NIMH cells SWELL when charged fully (ruining them) if you aren't careful to bolt them together similar to how toyota did it. Also a temperature sensor with charger power cutoff relay has worked well for me in the past, but really a better one that can monitor each and every cell for temp and voltage would be nicer ( I just don't have the time to do it all myself ).

The Truth Sets You Free !

Log in or register to post comments


Who's online

There are currently 0 users online.

Who's new

  • pkvgameshouse
  • toomanyscoots
  • daftarpokerqq
  • Jacupp
  • micarr

Customize This