Maha MH C808M Charger

[bcwang]

Tom,

Do you know how the conditioning algorithm works on this charger? Such as charge current, discharge current, any rest time in between, etc....

I'm asking because I have the 808m and I tried the conditioning feature, but it didn't work as I expected. First, it charged. Then, it discharged, but somehow all batteries started discharging at the same time. Sounds like too much a coincidence, maybe I took my eye off it too long. Later I saw some of the AAA cells switch from discharging to charging. I went to sleep, woke up 4 hours later and saw the AAA's were still charging. There's no way that could happen unless the second charge was at a much lower current than the specified 700ma. This confused me, I'm going to try doing it again but paying closer attention to what's going on. Only problem is it takes so long to do a cycle.

Also, I'm wondering if the charger rests after the initial charge before starting the discharge like mentioned on the website where I purchased this charger. I'm not sure if I can tell by looking at the charger to see if that happens, but maybe testing you did with your scopes and meters can tell you how it actually works.

Thanks for any info!

 

[wptski]

bcwang:

No answer for you but I did a quick ckeck this morning on my C808M. All positive and negative contacts are common to eachother! How does it know if it's a AAA installed or a AA, C or D cell?

 

[bcwang]

bcwang:

No answer for you but I did a quick ckeck this morning on my C808M. All positive and negative contacts are common to eachother! How does it know if it's a AAA installed or a AA, C or D cell?

If you notice under the aaa groove, there is a small button that gets depressed when you put in a AAA cell. This must signal to the charger that a AAA cell is inserted and use the 700ma and 350ma rate. All the other cell sizes charge at the same 2amp or 1 amp rate. I only wonder the consequences if that switch failed and a AAA cell is charged at 2 amps.

I tried the refresh again last night on 8 aaa cells, one of them I saw start discharging 1 hour into it since it was a pretty full battery. 9 hours later, all the batteries are charging still and not finished. I really wonder what discharge and charge current the refresh mode uses, especially on AAA batteries.

 

[bcwang]

After reading the review I thought that I'd have problems removing or inserting AA cells. I had no problem at all. I just used the same procedure that I've always used on all my chargers. Insert negative end first and push the positive end in. Just reverse to remove. No problem here!

Yes, the LCD icons could have been bigger and bolder.

I think the problems with cell removal were with the 801d, not the 808m. I have the 808m and it's extremely easy to remove aa and aaa cells since you have enough clearance to grab the entire cell.

The 801d has no spacing between cells to put your finger, so you can't easily remove cells sandwiched between other cells. If you start at the ends though, you can remove all the cells easily one by one because there are gaps at each end to get your finger underneath to grab the cells.

 

[wptski]

If you notice under the aaa groove, there is a small button that gets depressed when you put in a AAA cell. This must signal to the charger that a AAA cell is inserted and use the 700ma and 350ma rate. All the other cell sizes charge at the same 2amp or 1 amp rate. I only wonder the consequences if that switch failed and a AAA cell is charged at 2 amps.

I tried the refresh again last night on 8 aaa cells, one of them I saw start discharging 1 hour into it since it was a pretty full battery. 9 hours later, all the batteries are charging still and not finished. I really wonder what discharge and charge current the refresh mode uses, especially on AAA batteries.

bcwang:

Ah! Ha! I didn't notice the little button. If one could make the switch and move the cell outside the charger, current could be easily measured. I'll look into doing that

 

[wptski]

bcwang:

I had two AAA that came out of my wireless mouse. I defeated the switch with a piece of wooden taped down. Wires out to a AAA holder. My Fluke 189 shows 1.9A! What's wrong here? I put the other cell in the charger and watched it charge in 15 minutes and got to 93F!

I connected the other cell outside the charger again, 1.9A or so and got on the phone to MAHA. I should have caught this myself as I had a scope on it the other day(post above somewhere). It's a pulse charger and the current is the same for all cells depending on the mode. The switch changes the pulse rate much slower for AAA. It looks like at least two seconds. One could probably listen to the difference in clicks, comparing AAA's to the others. I'll scope it another time.

They said that the conditioning cycle uses the slower pulsed current for all three stages but I think that Tom was told differently?

 

[SilverFox]

Hello Bcwang,

I have noticed some strange things during conditioning as well. I have been in contact with Maha about it, but have not heard back from them.

I was discharging some C and D cells and half way through the discharge (by the way the discharge current is 250 mA EDIT I was off on this, it is more like 350 ma, but seel post #54 for the real numbers. END EDIT), I added some AA and AAA cells. The cells I added did not charge up first, but gave an indication of discharging. About four hours later, they still indicated that they were still discharging, and I still had the same two bars on the indicator.

When the first cells I put in had finished their discharge cycle, all the cells began charging. The AAA cells finished charging first, followed by the AA cells, then the C cells, and finally the D cells. In the end, everything ended up fully charge, but like you I noticed that it took a long time.

The conditioning algorithm is supposed to first charge the cells, then discharge them, then charge them back up. You have a choice of the two charging rates. If you put a cell in and push the condition button, you will charge at the higher rate. If you put a cell in, push the condition button, then push the soft charge button, you will charge at the lower rate.

Tom

 

[bcwang]

Bill,

I was looking at your scope output picture, but I don't really understand it. 114mv is .114v right, so does that mean 114mv over battery resting voltage? Can that be translated into current somehow? Also, though I know the period is 7.754ms I don't see a second pulse so can't determine the pulse rate from the graph. Help me understand the graph better.

It's interesting that the AAA switch only changes the pulse rate, and not the current. But 1.9amp seems too low for the charge current, you can't average that to be 2amps no matter how long the pulse is. From my observations, the click you hear with the charger can't be the pulse rate, as that is far too slow, especially if each charge pulse is only 7.754ms long. Maybe each click is when the charger re-measures the cell voltage to update the progress meter.

Tom,

The soft charge button does seem to affect the initial charge rate, but after the discharge, the charge rate doesn't seem the same. I will definitely try too test it a third time and try to really time each stage.

Do you know if AAA also use a 250ma discharge current? If it does then its odd that after 9 hours I couldn't finish conditioning a set of aaa 850mah cells set at the fast charge current. Even if they were empty at the start, it should take at most 6 hours to finish the entire cycle, maybe 7 max with worse case numbers for efficiency and cell capacity. 9 hours just doesn't make any sense with the data maha provides.

Another thing I noticed with conditioning, after I saw the cells in their final charging state, the conditioning signal went off. Any furthur cells inserted just charge like normal. I want to get to the bottom of how this thing works, it's still a bit baffling at the moment. I hope maha gets back to you soon an answer.

 

[wptski]

bcwang:

Yes, it's .114V higher than the resting voltage before the pulse and after each pulse the resting voltage is a bit higher and it keeps on increasing. The screen I posted is single event, the scope is waiting for a trigger pulse and catches one pulse only.

Well, the 1.9A pulsed isn't really like 1.9A constant current and the slower pulse for the AAA calculates to 700ma. The outcome is a cooler charge. The current was really more like 1.97A. If one wanted to compare, you could time the charge on another charger at 1.97A on cells in the same state of charge(SOC) and compare that to the C808M.

Your right about the clicks! It can't be the pulse rate, way too slow.

I'll try to capture some AAA charging screen and superimpose it with the AA screen.

I just got this scope last week and still learning although I've used automotive scopes, this one is a bit different.

 

[willchueh]

Hello everyone,

I work for Maha Energy and headup the strategic product group - more specifically, our group develops advanced charging algorithms. The ASIC in MH-C801D & MH-C808M was developed by several engineers in our group. I have been watching this thread for a while and thought I would clarify some of your questions:

1) Unlike most chargers, the MH-C808M (and MH-C801D) uses a dual-stage PWM (pulse-width management) circuit to maintain the constant charging current. The first stage PWM is a high-frequency one (several hundred KHz) and establishes a fixed 2.0A current. This is done all in the AC adapter. Then, based on the microcontroller in the charger, a second low-frequency (on the order of hertz) PWM controller regulates the 2.0A to modulate it to different currents by altering the duty cycle. For example, when you use Soft Charging, the current is 2.0A but the duty cycle is a little less than 50% resulting in an averaged current of ~1.0A. Similarly, when a AAA is detected, the current is still 2.0A but the duty cycle is only 1/3.

So if you use a ammeter to measure the current, make sure it is time averaged (otherwise you will see the meter oscillate between 0 and 2A). The best way is to use a current-probe on a scope.

2) The MH-C808M uses a very intelligent conditioning algorithm. In a nutshell, the charger will let the batteries to rest until all batteries are charged before going to discharge. Similarly, the charger will wait until all the batteries are discharged before going to charge. While charging, however, the charger will show the next step (ie. in the initial charge, two batteries might show charge and 6 batteries show discharge - the six batteries are actually "standing by" for discharge. Discharge commences only when all batteries have finished charging.) The purpose of this is to allow for sufficient time to elapse between charge-discharge without waiting too long. Many designers omit the rest time, but is critical for maximizing battery life cycles.

This is the reason behind the perception of a "long" charge during the conditioning cycle. The batteries actually don't start charging (despite LCD display) until all batteries are discharged. You can detect the moment the charger starts charging when all the CHARGE indicators are on for the first time.

We didn't want to clutter the already-small LCD with another "REST" symbol so this was the compromise- sorry for the confusion!

3) The AAA discharge current is at 1/3 duty cycle. If Soft Charging and Conditioning is enabled simultaneously, the discharge (and charge) current is at 1/2 duty cycle for AA and 1/6 duty cycle for AAA.


William
Maha Energy (North America) Corp.

 

[wptski]

William:

Welcome to CPF! I asked for you today when I called but you were away from your desk. We spoken several times over the years. Thanks for explaining the operation of the C808M and C801D. I shall explore it's operation with a scope a bit deeper in the near future.

 

[BVH]

William, it is really great when a manufacturer's representative actually takes the time to come here to CPF and participates in the threads. Thank you very, very much. I love my 808!

 

[bcwang]

Hi William,

Glad you could join us here and provide more info for this charger. Let me see if I understand everything.

Charge Currents (even in conditioning mode):
AA, C, D - Normal Charge - 2 amp constant current
AA, C, D - Soft Charge - 2 amp pulsed at a 50% duty cycle = 1 amp average current
AAA - Normal Charge - 2 amp pulsed at a 35% duty cycle = 700ma average current
AAA - Soft Charge - 2 amp pulsed at a 17% duty cycle = 350ma average current

Conditioning Discharge Currents: ( I think I got all this wrong, but trying to just follow the info you gave plus what I know from other sources)
AA, C, D - normal conditioning - 250ma constant current
AA, C, D - soft conditioning - 125ma constant current
AAA - normal conditioning - 233ma constant current
AAA - soft conditioning - 117ma constant current

Maintanence Trickle Current:
AA, AAA, C, D - all modes - 2amp at 0.4% duty cycle for average of 8ma average

A few questions:

1. In the conditioning mode if only one battery, or if all batteries are identical in capacity and state of charge (so they all end at the same time), is there still a rest period between charging->discharging, and between discharging->charging?

2. What happens if new cells are inserted, or cells taken out and replaced while in the middle of the conditioning cycle? Does that cell just follow in same phase of conditioning all the other cells are in instead of starting it's own conditioning cycle from the beginning? Based on my observations and what Tom mentioned, this seems to be the case.

3. The manual mentions putting batteries in from left to right and not skipping any slots. It also says to put higher capacity batteries on the left. This is very odd for an independent cell charger. Tom tested it and showed no difference in performance from the different slots, so is there any reason this statement is in the manual? It's actually mentioned quite clearly as a warning message almost, and there are multiple diagrams showing this as how to "correctly" load the batteries. (Interestingly, many of the press photos of this charger show the aaa cells on the left and the D cells on the right, exactly opposite to how the manual recommends battery loading)

My constructive feedback
-The area around the last two slots get warm from some charger circuit, even when not occupied by batteries. The rest of the charger is cold, so maybe that circuitry could have been in the front of the charger or somewhere else where it wouldn't affect cell temperature at all.

-Discharge current quite low, if 250ma is the fastest discharge current, it will take forever to discharge a D cell, even for AA cells I get impatient waiting so long. The C204W discharges at 400ma, and it's a 1 year older charger.

I'm pretty impressed with each new generation of Maha chargers, and this one is the ultimate one yet. I hope it's the last charger I buy for a while. (I've got way too many chargers, says my wife)

By the way William, would I be able to ask you questions on the MH-C204W maybe outside of this thread? I've got tons of questions about that charger as well.

 

[willchueh]

Hi William,

Glad you could join us here and provide more info for this charger. Let me see if I understand everything...

hello bcwang,

I am happy to answer your questions.

1) This is the summary of the currents:

AA, C, D - Normal charge - 2A
AA, C, D - Soft Charge - 2A x 1/2 duty cycle = 1.0A
AAA - Normal charge - 2A x 1/3 duty cycle = 0.66A
AAA - Soft charge - 2A x 1/6 duty cycle = 0.33A

This is the rough charging currents. Actual charging currents are little lower as the charger applies "special" pulses and very brief "pauses". Typical charging current are 95% of these values.

Conditioning current:

AA, C, D - normal - 360mA
AA, C, D - soft charge - 360mA x 1/2 duty cycle = 180mA
AAA - normal - 360mA x 1/3 = 120mA
AAA - soft - 360x 1/6 = 60mA

Again, the charger has special pulses and pauses during the discharging process designed to enhance battery performance. Actual discharging current are around 90-93% of these values.

2) If the batteries are identical in charge, there will be no waiting period. This was a design compromise. If we always wait 2 or 3 hours, the total charging time would be too long. So we just made batteries wait for the last one to finish. Granted the last battery wouldn't get the rest, but it also should have the least heat as the other batteries (contributing heat) have already finished discharging.

3) If new batteries are inserted during discharge, for example, it will skip initial charging and goes into discharge with the rest of the batteries. Similarly, if you insert batteries during the recharging part of conditioning, it will skip discharging and go the charging.

Hope this info helps! Feel free to email me with questions on other products, or start another thread!

William

 

[willchueh]

Many of you are probably wondering WHY Maha decided not to let the MH-C808M charge and discharge different banks at the same time (after all it is suppose to be an independent charger). From first inspection, it does seem like a total waste of time, having one battery wait for another to finish discharging! Other chargers discussed here (like the BC-900) and other Maha chargers can independently charge & discharge.

Here is why...

When a battery is discharged, all of the energy is dissapated as heat through transistors or resistors. If we are discharging 3 NiMH battery at 1A, that's 3W. For those of you familiar with the temperature of a transistor disspating 3W, you know how hot that is given the small size of most chargers.

So we have recently found out charging some batteries and discharging others at the same time is bad for battery health. The heat dissapated by the batteries being discharged will heat up the batteries being charged. Batteries can already get pretty warm during charge ... imagine adding another 3W on top of that.

William

 

[jclarksnakes]

....This forum is great. I had been running some batteries through conditioning cycle on my 801 and noticed many of the things others have posted here. I was getting what seemed like strange discharge and recharge times. I came back to this thread to ask questions but THANKS to William they are already answered. It is very impressive that the guy from the factory is here on the forum to answer our questions.

...Thanks again to William and to Tom and to everyone else who knows more than I do about the subject and has participated in this thread.

jc

 

[bcwang]

Hi William,

Thanks for your information about the charge and discharge currents, this helps explain a lot of the behavior some of us are seeing. I didn't know discharge current differed based on soft mode or not, or being a AAA battery or not. Knowing the discharge current based on the setting makes the long conditioning times some of us are seeing make much more sense. I guess the manual isn't entirely accurate (charge currents don't match what you mentioned) or fully informative (missing discharge currents) on the specifications on the charger.

I'm still wondering about why the manual states:

-ALWAYS INSERT LARGER BATTERIES FIRST STARTING
FROM THE LEFT (e.g. D batteries in slot 1, 2, C batteries in slot
3, 4 and AA batteries in 5, 6, 7, 8).

-ALWAYS INSERT THE BATTERIES FROM LEFT TO RIGHT
LEAVING NO EMPTY SLOT.

- For best performance, group batteries of same size and/or
similar charge together.

Those statements were the first thing to worry me about this indepedent cell charger. I definitely didn't want charging performance to suffer if I didn't follow those rules, but I also want to be able to pull cells in and out at random from the charger without affecting the completeness of charge. Any reason the manual spends so much time dwelling over these, even including diagrams showing how to do it correctly?

By the way thanks for all the help so far, it's been great to have someone from Maha helping us all out here.

Ben

 

[willchueh]

Ben,

With the MH-C808M, the instructions are designed to minimize the heat batteries exposed to. As you have noted, the right-side of the charger is warmer so we recommend to insert batteries from the left.

But as Tom mentioned, not following this doesn't have any effect on the performance of the MH-C808M.

So feel free to charge the batteries in any combination, take out charged batteries and replace with new ones!

cheers,

William

 

[john2551]

Hello William,

I currently use an Ansmann Energy-16 charger that has 16 completely indepedent charging circuits. I have seen them all work independently, so i know this is the case. My question is if i have four 12000mah D cells charging in a C808M charger for almost 8-9 hours at the 2amp rate & they are almost completely charged & then i put in one or two more almost depleted 12000mah D cells, will this cause the almost charged cells to go through another whole complete cycle & in turn another wait of 8 or 9 hours for the cells that were already in there 8-9 hours already?

Thanks,

John

 

[willchueh]

My question is if i have four 12000mah D cells charging in a C808M charger for almost 8-9 hours at the 2amp rate & they are almost completely charged & then i put in one or two more almost depleted 12000mah D cells, will this cause the almost charged cells to go through another whole complete cycle & in turn another wait of 8 or 9 hours for the cells that were already in there 8-9 hours already?

John,

The MH-C808M would "wait" for other batteries ONLY in the CONDITION mode listed below:

1) In the CONDITION mode, the charger will wait for all batteries to finish charging before entering discharge (condition mode is charge-discharge-recharge).

2) In the CONDITION mode, the charger will wait for all batteries to finish discharging before entering the recharging mode.

Therefore, if you are not using the CONDITION mode, there is no waiting involved.

Say you put in 7 12000mAh batteries for 6 hours (BTW the charging time is more liky 7 hours for these, not 9), and insert one completely discharged 12000mAh, the first seven will terminate in about an hours while the 8th one will terminate in another 7 hours.

In charging mode, everything is independent. Practically speaking, it's 8 separate chargers.

Hope this info helps!

PS: see earlier post why we are doing this.

William