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All Forums >> Radios, Batteries, Clubhouse and more >> Batteries & Chargers >> Charger Theory and Mathematics
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Charger Theory and Mathematics - 10/28/2002 3:30:10 AM   
ctdahle



Posts: 250
Joined: 12/3/2001
From: Del Norte, CO, USA
Status: offline 		Gentlemen,

Please check my math and my theory if you would be so kind.

I recently became tired of the forest of walwarts, powerstrips and lamp timers strung about my shop for battery charging and bought a Hobbico Multi-Charger.

I discovered the hard way that plugged in to the timer, the multicharger discharges when the timer is on the off part of the cycle and am fixing that problem with a bunch of diodes. (Would have been nice if Hobbico had mentioned that in the instructions)

I am also bothered about matching the chargers available rates to my various flight packs. I have 250, 400, 600, 700, and 900 MAH packs, and the charger will deliver 10, 25, 50, 100, 240, and 500 MA charging currents.

I understand that the general rule of thumb is to charge at 1/10C for a period of time sufficient to deliver about 120% of the pack's rated capacity.

Since the charger's power settings do not match the capacity of my packs, I am assuming that the wise plan is not to exceed 1/10C, therefore I am stepping the rate down to the next lowest rate and increasing the charge time accordingly. Thus, a fully discharged 700MAH pack charged at 50MAH needs to stay on the charger for 16.8 hours.

Thoughts, ideas, bouquets, brickbats?

_____________________________

When I get a little money I buy model airplanes and if there is any left over, I buy food.
       Post #: 1

Charger Theory and Mathematics - 10/29/2002 11:16:29 PM   
mulligan



Posts: 1147
Joined: 4/14/2002
From: Sanford, FL, USA
Status: offline 		The Hobbico Multi-Charger does NOT put out the current it says. Mine puts out anywhere between 20 - 50% more current than specified. And since it is completely unintelligent, it is little more than four wall warts wrapped into one box, albeit with different ratings. I don't even use a timer, because of the inaccurate current ratings- I have to monitor voltage until it hits 1.4 - 1.43 V per cell. If you really want to simplify your charging, you'll need to pony up for an automatic, digital charger/cycler.

As far as charge rates go, you can go up to 1/1 with NiMHs and even higher for NiCds. However, if you go over 1/10, the cells may not tolerate overcharging well (cell venting, loss of capacity).

Typically, the charge cycle efficiency is such that you need to charge for 50% longer than the math would suggest. For example:

For 500mAh pack at 50mA: 15 hrs.
For 500mAh pack at 500mA: 1.5 hrs.
For 900mAh pack at 500mA: 2.7 hrs.
Not that these times are approximate FULL CHARGE times (cell was completely discharged at beginning of charge) and vary with the pack and ESPECIALLY if the charge rates are not exact. I suggest you monitor voltages on a couple of charge cycles to determine what your times should be and go from there.

Regards,
George

(in reply to ctdahle)
       Post #: 2

Charger Theory and Mathematics - 10/30/2002 7:48:15 AM   
ctdahle



Posts: 250
Joined: 12/3/2001
From: Del Norte, CO, USA
Status: offline 		Thanks for the comments.

Are you saying that to fully charge (from a discharged state) that I really should be "putting in" the equivalent of 150% of capacity?

The thing is, I have been using the "hour a day" timer and wallwart charging system since I bought my first radio in 1974, and have had only one battery failure, which was not charger related.

At the moment I'm kinda thinking I ought not have tried to fix what ain't broke, and am about half tempted to send the Multicharger back.

The Intermatic timers, powerstrips, wallwarts and the spaghetti factory of charging leads all over the bench may be a pain, but having marked all the leads with various colors of heat shrink tape and having retro-fit them all with banana plugs and sockets and all, maybe I ought to leave well enough alone.

I really thought the Multicharger was just a glorified wallwart. I am disappointed to find out that actually, it is less than a wallwart, albiet one with selectable ranges and a nice, clean looking panel.

_____________________________

When I get a little money I buy model airplanes and if there is any left over, I buy food.

(in reply to ctdahle)
       Post #: 3

Charger Theory and Mathematics - 10/30/2002 9:08:30 AM   
thomasb



Posts: 343
Joined: 6/6/2002
From: Private, CA, USA
Status: offline 		[QUOTE]I understand that the general rule of thumb is to charge at 1/10C for a period of time sufficient to deliver about 120% of the pack's rated capacity. [/QUOTE]To cover worst case slow charge efficiencies, 140% to 160% is nominally used. These values correspond to the 14-16 hour charge times when C/10 is used.

The mentioned 150% is fine too. It falls nicely between the two data points. Higher charge currents than C/10 should not be done on a timed basis -- use a neg delta peak charger for that.

(in reply to ctdahle)
       Post #: 4

Charger Theory and Mathematics - 10/30/2002 10:02:57 AM   
ctdahle



Posts: 250
Joined: 12/3/2001
From: Del Norte, CO, USA
Status: offline 		OK, so decision time...

Clearly the Multi-charger puts a load on the batteries if it's power is cut off but the packs are left connected. This by itself indicates against using the "Red Scholefeld lamp timer method" to maintain batteries between flying sessions unless I stick with the factory WallWarts.

But, that problem can be eliminated if I build a set of diode protected charge leads, or better yet a set of diode protected jumpers to use with my existing leads as needed.

So, the question is, if I build the leads, can I safely use the Multicharger, connected to a timer to charge and maintain three flight packs and a TX, with the timer set to an hour a day maintainence charge, assuming a rate about 1/3 less than 1/10 c?

If so, figuring that I put a 250-300 Ma load on a 700 Mah pack for about 2 hours, how long should I set the timer for when I get home from the field, assuming I set the charger to it's 50MA setting?

Put another way, given that charge rates higher than 1/10c MUST be carefully monitored to avoid burning up the cells, at what rate less than 1/10c does an intermittent (i.e. one hour daily) trickle charge become ineffective to maintain the battery pack in it's ready state?

In the alternative, would it be better to set the charger to it's 100Mah setting for say 10 hours for the 700 Mah pack, and then let it maintain at that same rate for one hour daily, or would that rate be enough to fry the cells over a few month's time?

My goal is to plug the batteries into the chargers when I get home from flying on sunday afternoon, set the timer to an appropriate time and forget about it until the following saturday morning.

The last thing I want to have to do is to remember to either charge the batteries friday night, or to have to roll out of bed at 3:00am to disconnect the batteries from the chargers.

_____________________________

When I get a little money I buy model airplanes and if there is any left over, I buy food.

(in reply to ctdahle)
       Post #: 5

Charger Theory and Mathematics - 10/30/2002 11:05:23 AM   
thomasb



Posts: 343
Joined: 6/6/2002
From: Private, CA, USA
Status: offline 		[QUOTE]So, the question is, if I build the leads, can I safely use the Multicharger, connected to a timer to charge and maintain three flight packs and a TX, with the timer set to an hour a day maintenance charge, assuming a rate about 1/3 less than 1/10 c? [/QUOTE]Yes. That would work out to about a 3.5% of capacity per hour, enough to cover the 1-3% per day loss that a "healthy" pack may experience from self discharge.

[QUOTE]If so, figuring that I put a 250-300 Ma load on a 700 Mah pack for about 2 hours, how long should I set the timer for when I get home from the field, assuming I set the charger to it's 50MA setting? [/QUOTE]Don't dwell on splitting hairs. Stick to a regiment where you just perform a c/10 for 14 hours. This is a better habit to follow and the pack will NOT be damaged from it.

[QUOTE]Put another way, given that charge rates higher than 1/10c MUST be carefully monitored to avoid burning up the cells, at what rate less than 1/10c does an intermittent (i.e. one hour daily) trickle charge become ineffective to maintain the battery pack in it's ready state? [/QUOTE]When it puts less than 4% back into the pack per day.

[QUOTE]In the alternative, would it be better to set the charger to it's 100Mah setting for say 10 hours for the 700 Mah pack, and then let it maintain at that same rate for one hour daily, or would that rate be enough to fry the cells over a few month's time? [/QUOTE]If you are sure that the charger is putting out only 100mA, then this should be OK.

You may be able to tailor the current to a near-C/10 value. Chargers that are not constant current designs can be current limited with a series connected resistor. Try about 47 ohms 1/2 watt and see what happens.

(in reply to ctdahle)
       Post #: 6

ctdahle_Off Topic - 10/30/2002 11:41:05 AM   
bob_nj


 

Posts: 3472
Joined: 12/3/2001
From: Vineland, NJ, USA
Status: offline 		Haven't seen that username for quite awhile! If I remember correctly, we used to hang out religiously at the OLD old RCO. It was a nice place at one time. How have you been doing, and what kind of flying are you into now_bob

(in reply to ctdahle)
       Post #: 7

Charger Theory and Mathematics - 10/30/2002 7:35:12 PM   
ed42d-RCU



Posts: 171
Joined: 8/2/2002
From: Buffalo Grove, IL, USA
Status: offline 		ctdahle,

I just bought a timer to try the "one-hour a day" charge. I believe I experenced the same problem with my Hobbico Multi-charger using a timer. If the timer is in the off position, I noticed the Multi-charger light was on to the lead that was connected to my receiver pack. Thinking there was something wrong with my cables, I made up a new cable with the same results. Is this the same problem that you are seeing?

I also noticed that my transmitters do not exhibit this problem. I don't think the transmitter has a diode because I can cycle the batteries on my digipace.

I have abandoned trying to use the "one-hour a day" charge because of this problem.

(in reply to ctdahle)
       Post #: 8

Charger Theory and Mathematics - 10/31/2002 5:44:05 AM   
wrbirch


 

Posts: 108
Joined: 6/8/2002
From: akron, OH, USA
Status: offline 		your diods will have aprox. .6 vold drop across them and might stop the charger from fully charging your bats or cause it to take longer than calculations. also charger won't be able to monitor bat voltage because thediods will block it from reaching charger

(in reply to ctdahle)
       Post #: 9

Charger Theory and Mathematics - 10/31/2002 6:34:30 AM   
ctdahle



Posts: 250
Joined: 12/3/2001
From: Del Norte, CO, USA
Status: offline 		Hi Bob, I do remember you from the "golden age" of RCO. Since I decided to start posting here, I have run into a lot of old RCO friends but rather than drive this thread off topic I guess I will give everybody an update over in The Clubhouse.

Back to the subject of charging...

I am going to make up the diode protected leads and give the timer method a try.

Thomas, I appreciate your comments about just establishing a regimen. That is exactly what I had before I bought this new charger, and that is what I want to get back to. I just don't want to set up a regimen that will fry the batteries.

To the rest, If I fry my batteries or lose a plane, I will sure keep you all advised.

Mr. Birch, I'm not sure that the multicharger monitors voltage in any meaningful way, but based on your concern, before I go all out making up four sets of trick jumpers and all I'm going to breadboard something together to see how it all works.

Ed, my experience with the timer method and radio manufacturer's wall warts has been excellent for nearly 30 years, and with just one or two airplanes, there would not really be a reason to depart from it. As you note, it is the Hobbico Multicharger that screws up the program. Personally I would be sticking with the Wallwarts, but the recent arrival of a baby has banished my airplanes from my once capacious shop to a corner of the garage. Storing the wallwarts away is just one of the things I need to do to make the models take up less space. I had been using an entire bedroom for a shop, and had storage stations set up for two sets of flightline gear and four line ready airplanes. Now the shop is a nursery full of teddy bears and diapers...

Anyway, I think I can make the Multicharger do the job of seven of the Wall Warts and that is worthwhile increase in efficiency.

_____________________________

When I get a little money I buy model airplanes and if there is any left over, I buy food.

(in reply to ctdahle)
       Post #: 10

Charger Theory and Mathematics - 10/31/2002 7:06:24 AM   
thomasb



Posts: 343
Joined: 6/6/2002
From: Private, CA, USA
Status: offline 		My money is on the diode trick. Only one diode is required per output. Any of the 1N4000 series will work.

(in reply to ctdahle)
       Post #: 11

Another option - 10/31/2002 10:14:40 PM   
JPMacG


 

Posts: 1138
Joined: 1/2/2002
From: Ivyland, PA, USA
Status: offline 		Hello CTDahle

Here is another option: A constant current source can be constructed very easily using an LM317 regulator IC and a resistor. The value of the resistor controls the charging current and can be chosen to provide the current you desire for a particular battey pack. The parts cost a few pennies if you buy from Mouser or DigiKey and a few dollars from Radio Shack.

A single DC supply (big wallwart) could provide the current for all your charging. Each battery pack would need to have its own regulartor and resistor, which would be customized for the required charging current of that pack. The regulators are connected in parallel to the same DC supply.

I have done this myself and it works well. It gives me the freedom to set the current as I wish.

A series resistor shunted by an LED in the lead to the battery lets me know that the battery is acutally charging, e.g. the plug has made contact.
Depending upon your power supply's voltage and the charging current, the regulator may require a heat sink.

Information and circuits are described on various RC sites on the internet. One site is http://www.uoguelph.ca/~antoon/gadgets/gadgets.htm Click on the "Simple Constant Current Source" by John Nooyen. If you have a DC supply you won't need the bridge rectifier and capacitor he shows on the left side of the circuit.

< Message edited by JPMacG -- Oct 31 2002 5:22PM >

(in reply to ctdahle)
       Post #: 12

Charger Theory and Mathematics - 11/1/2002 7:21:24 AM   
ctdahle



Posts: 250
Joined: 12/3/2001
From: Del Norte, CO, USA
Status: offline 		Thomas, Thanks for the info on the diodes. Hopefully I will make it into the "big city" (Monte Vista, 3500 souls, big enough for a Radio Shack) on Saturday. I took the cover off the charger to see if I could put the diodes inside the case some how, but it is not realistic to do so. I'm just going to make up a pigtail for each output.

JP, I like your idea, and I should have done something along those lines before I bought the multicharger. I hope someone else can get some mileage out of your suggestion. Thanks for the link.

_____________________________

When I get a little money I buy model airplanes and if there is any left over, I buy food.

(in reply to ctdahle)
       Post #: 13

Charger Theory and Mathematics - 11/2/2002 10:09:07 PM   
hebertjj


 

Posts: 323
Joined: 1/18/2002
From: Knoxville, TN, USA
Status: offline 		Good Grief Guys,

Do you all fly R/C or charge batteries and make leads? This thread sounds like the Amish net site where they discuss the pro and cons of disk versus drum brakes on their buggies!

Have any of you all heard of peak detection charging? If you don't want to use one of these, and even if you do, for "leave it on all the time" charging, get a timer set to 1 or 2 hrs a day, a power strip, and plug all your wall warts into it and leave you packs pluged up to this all the time. Otherwise use the peak charger right before going to the field. Any diodes put in the lines will lower the voltage at the pack and lower the charge rate.

Joel

(in reply to ctdahle)