Battery Charger circuit for my bot.

morris4019

I recently purchased some "AA" cell batteries (with solder tabs) to create a few battery packs for my robot. The plan is to initially design a seperate battery charger so that i can charge my packs. Then later i will also add the designed circuit in to the PCBs design so that i can just plug the robot in if needing a charge.

I will be connecting the batteries in 3packs, 3batts x 1.2v = 3.6v. The mAh rating for my particular batteries are 1000mAh per "AA" cell, so the packs should be about 3000mAh rating. I'm just not sure how to begin designing a battery charger. Have seen designs using a PIC, but i have 2 extra 40pin Propellers stored away which i thought i would use instead, or a BS2, i figured either one would do the trick, I just don't quite understand how to, when the battery gets to full charge, or near full charge, stop and put in trickle mode.

-Mike

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Mike Green

1) When you put batteries in series, the mAh stays the same (the voltages add). You don't want to put batteries in parallel without some way to prevent one battery from discharging through another because batteries are not absolutely identical and there will always be one that has a slightly higher voltage and that one will discharge through a lower voltage one.

2) I assume these batteries are NiMH. You need a different sort of charger for each battery chemistry. Do a Google search for "DIY NiMH Charger" and you'll find a lot of suggestions for chargers. Maxim makes charger chips and has datasheets on their website.

3) The various battery manufacturers have recommended charging criteria on their websites. Maxim's datasheet (MAX712) also discusses charging (and charge cutoff) criteria.

Post Edited (Mike Green) : 7/15/2009 4:11:49 PM GMT

morris4019

I thought the mAh added as well, i guess one more trip to the drawing board is my next step.

Thanks Mike, i have ordered a few MAX713 (because the batteries i chose were NiCad). My project is requiring at anytime at least 1.5Ah (1500mAh), which i thought would have been met, but i guess i was mistaken about the battery packs. If the mAh don't add, like i thought, then i will be .5A short of my current requirement.

-Michael

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LilDi

Duracell AA-NiMH rechargable batteries are rated at 2650mAh

Mike Green

Remember that the mAh is the current supplied times the number of hours that current is drawn.· A 1000 mAh cell can provide 1.5A for about 40 minutes before it's exhausted.· You can parallel battery packs if you put a diode in series with the battery to prevent one pack from trying to charge another.· You'll lose a little bit of power (0.7V x <the current drawn>), but you can add an extra cell to each battery pack to make up for that.

What are you running off these batteries and what kinds of voltages are needed?

I recommend using a switching regulator for the robot and relatively high battery voltages (7.2V, 9.6V, 12V, etc.)· This is much more efficient than trying to use low voltages and linear low drop-out regulators (3.6V, 4.8V, 6.0V, etc.)

morris4019

My main concern was that batteries in todays market, in the US at least, are very expensive. The idea is lost, "Disposable Batteries", when a 4pack of "AAA"s is $8-10 depending on where you go. Also, buying battery packs, at least where i had originally researched, were very expensive too. I decided i would try building my own packs, and when i found good deals i bought them up. Although i was mistaken, and thanks for pointing it out Mike [noparse]:)[/noparse], that the voltages would be added but not the capacity. I am not going to return the batteries i have bought, i can·definately use them for something else. But before i buy more i figured i would ask you guys first. I found these battery packs:

http://www.all-battery.com/6v2300mahnimhsidebysiderxreceiverbatterypackswhitecconnectorforrcaircraftswalkingrobot-1-2.aspx

Which are very inexpensive and should work perfect for what i'm doing. And what is i'm doing? An overhaul of sorts on my·tracked robot. It's running propeller, IR detection (emitters use 3.3v/detector uses 5v), 3 general purpose LEDs for status and detection, 2·x motors·GM9 (solorbotics,·rated 3v=400mA,·5v=600mA). I was having alot of problems using an L298·controlling these motors, and i believe alot of it was the fact that my battery selection was·not right. But now that i am using L293d (much easier, and perfectly rated for my project), it's alot easier.·I was originally going to use 2x 3"AA" cell packs, which would give me·7.2v. With the batteries that i bought i would need·2x·6"AA" cell packs to just be at 2Ah, correct? But the packs that i found above, which you can't beat the price, are·3"AA" cell 2300mah packs, which would be perfect, right?

Again, sorry for all the dumb·questions, but i'm finnaly starting to understand some of this and i get very curious, and sometimes very impatient and buy things without fully understanding what i'm doing.

-Michael Morris

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morris4019

I also found these, which would work pretty good, but I guess i would still have the problem of having the motors and electronics on the same battery.

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1330

http://www.all-battery.com/li-ion18650148v2200mahrechargeablebatterypackwithpcbprotection.aspx

the last one is a Li-Ion 18650 14.8V 2200 mAh Rechargeable Battery Pack with PCB Protection which would play nice with my small design, only being the size of 4 "AA" batteries.

Do any of these sound like they would work out alright? Or should i continue to plan on using 2 seperate packs, one for motors, one for other electronics.

-Michael Morris

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Prettybird

· They do make smart chargers that can handle from 2.4V to 7.2V automaticly. I have 3 different voltage battery packs on my bot. I tried the 6v on a 7.2 volt charger once and they got hot. Live and learn.

Mike Green

DO NOT use the Li-Ion battery packs unless you buy a charger made for that battery pack. Li-Ion batteries are very very fussy about how they're charged and will violently catch fire if charged improperly or discharged improperly. The "PCB Protection" included with your battery will help, but unless you're very experienced in the "care and feeding" of Li-Ion batteries, do not try to homebrew or cut corners. These batteries are very light, have great characteristics except that they catch fire and explode when mishandled.

Beau Schwabe

Just something to consider... for NiMh, NiCd, SLA, LiFePO4, NiZn, etc. If your power charging source (aka Solar Cells, or some wall-wart) can not keep up with the "normal" charge cycle demands i.e. Constant current and/or Constant voltage, then most all of your charging options will be reduced to a "safe" trickle charge mode dictated by the manufacturers specifications.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

CounterRotatingProps

speaking of Li-ion - I'm wondering if there's a reference-standard like charging circuit on the 'net --- for what ever reason, there seems to be a paucity of these schematics.

what do you all think of this for Li-Ion charging - it's not mine, nor can I vouch for it, just wondering if it's safe and reasonable as it can be used for several types of batts (according to the author):

http://www.bowdenshobbycircuits.info/page12.htm#lithium.gif

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morris4019

Yes i had planned, if i bought the Li-ion, to use the Universal Li-ion charger that they recommend. this is the link to it:

TLP-2000 Universal Smart Charger for Li-Ion/Polymer battery Pack (3.7V-14.8V 1-4 cells)
http://www.all-battery.com/universalsmartchargerforli-ionpolymerbatterypack37v-148v1-4cells.aspx

As far as discharging, it says 2.5C (5A), which would in my case probably never dicharge over 1.5A, and definately never over 2A. I would probably nix the idea of an onboard charger, if i used the li-ion batteris, so that a safe charge could be produced from the Universal Smart Charger, for safety.

-Michael Morris

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W9GFO

I highly recommend getting a good battery charger. I have several and I use at least one of them every few days.

For charging LiPos I use the Thunder Power TP-1010C along with the Thunder Power TP-210V Balancer. I will also sometimes use the Great Planes Triton for LiPos that don't have the balancing taps.

For high cell count (up to 40) NiMh or NiCads I use the AstroFlight 112 Deluxe.

For 6-7 cell RC Car packs I use the Duratrax Intellipeak Twin Pulse Charger.

My Favorite and easiest to use is the Dymond Turbo Super Charger. No Longer on Dymond's website. Looks like their Delta X 6 Plus has replaced it.

For AA's I use the Maha MH-C800S Eight Cell Smart Charger.

I also use a 16 year old Litco Alpha 4 from time to time.





Rich H

Post Edited (W9GFO) : 7/16/2009 4:58:52 PM GMT

W9GFO

I have hundreds of Saft 3000 mah NiMh Sub C cells. You can have some (60) if you want. The catch -you need to pick them up and they will need to be removed from a Segway Battery Pack. They have much lower internal resistance than the AA's - more appropriate for high currents.

Rich H

Post Edited (W9GFO) : 7/16/2009 5:17:38 PM GMT

morris4019

I've decided to go against creating my own battery pack, because to get the voltage and capactiy i need, i will be spending just as much as buying what i need premade. Depending on the route i go i will be using a rechargable battery, but i will let some other person/company make them. I think i will go with the Li-Ion battery and possibly a different charger from the one i listed above. The only reason i would go back to that charger is because it is the recommended charger on their website.

Also, like Mike Green was saying above, I would like to use higher voltages, preferable 9v (which is where my motors would truly be perfect speed and torque), but they will push a little over an amp each. Which will mean that i will still be running my batteries at over 50-60% capactiy leaving me with very short battery life. On the other hand i could just add a bigger pack... but where? there is just no room to keep getting bigger and bigger batteries. That is why i am settleing wiht 5v-6v running to my motors, along with the 3.3 for prop and 5v for ir detection and possible using the Li-Ion. I currently have 12 "AA"s, yes 12, the wieght alone makes the bot barely move even with tracks. the Li-ion will be the size of 4AA batteries, maybe a littel bit bigger but that is definately doable, no worries on weight, no worries on room.

-Michael Morris

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W9GFO

Keep in mind that in addition to the strict charging requirements of LiPos, you must also be sure to not let them discharge below a certain level. They will be damaged by over discharging, more-so than other chemistries.

You can get low voltage alarms/cutoffs commercially or build your own.

Rich H

morris4019

Features and Benefit

• High quality 14.8 V Li-Ion rechargeable battery pack
• Made of·Four 2200mAh cylindrical 18650 cells with PCB and poly switch for full protection
• Light weight and higher energy density than any rechargeable battery
• No memory effect and rechargeable
• Longer storage life than NiMH battery
• You can build 14.8V/4.4Ah Li-Ion battery pack by connecting two PCB ready battery modules in parallel .·
• Built-in IC chip will prevent battery pack from·over charge and over discharge and prolongs battery life
• Ideal for replacing batteries for 12V HID (10 to 13W) and Diving Lights for long running time
• Perfect for building 14.8V battery pack for bike lighting, RC toy, robots,·laptop and DVD external battery.
• Use Smart Universal Li-ion Battery Charger for proper charging.

Is the IC chip that is built in not suppose to prevent battery pack over charge and over discharge? Or is the "Over Discharge" prevention just to keep it under the 5A discharge rate?

-Michael Morris

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Mike Green

The IC is supposed to prevent overcharge and over discharge, both rate (maximum current) and capacity (mAh - usually sensed by voltage threshold).

morris4019

ok, so I would have to then monitor the voltage in from the battery and make sure that it does not go below the cutoff voltage, or a little higher than the cutoff voltage to be safe, right?

It was my understanding, and hopefully i'm in the ball park here, that the battery voltage will stay pretty much at whatever it is rated (say 9v) until most all of the capacity is discharged, then you get a very fast drop from rated voltage. So if the 9v battery was almost fully discharged and the cutoff voltage was say 6v, or something, theoretically to be safe i should monitor the voltage at all time and have an override built in to shut down at about 7v correct?

-Michael Morris

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Prettybird

I have some data if interestted. I am using standard 1500mah ni cad battery (old) to run 2 parallax overloaded servos along with a ping and a seperate 6v nicad pack for all te electronics and leds. My bot was still crawling around at 2 hrs. but noticed slowdown after the servo battery power dropped to around 7.2 volts. It also carrys a panel digital meter for easy testing for now. Yes Voltage regulators are inexpensive and a easy way to keep constant voltage as the batterys drain. That is why you see them on electronics boards. 7.2v seems to be a standard with alot of radio controlled hobby toys. They have alot of different ratings depending on the mfgr. The plugs are all standard too.

morris4019

I have regulators, that is the question though. I have been told many times that I should be using a seperate battery source for the the motors and the electronics. In my mind, I guess i still don't understand why i would need this if, the total amount of current needed, is under half of what the batteries will produce. Say if i need 1.3A and my batteries produce 2.3Ah, i should be good for roughly 2 hours correct?

-Michael Morris

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Prettybird

I am not much on the calculating part but roughly guess. The nicads are used and as rechargable batterys are old from some radio controlled cars. As rechargable batterys age, they slowly decompose so I am sure they were stronger when new. Also running motors (especially brush motors) can throw noise in the supply and make computer stuff go crazy. Some processors are more sensitive then others. That is why you see alot of capacators and clamping diodes around inductive loads. Depending on your motors is how long they will last. Some of the faster R/C cars only can run 15 to 20 Min..As I said I was using extremely overloaded servo motors. Next test run, mabe I will try to measure the current under load (roaming). From what I have read in this forum a good quality 9v battery can run a stamp board over 40 hrs. if not loaded down with sensors, LEDs and other curcuitry. I know a 5 pack of AA nicads will handle the electronics for days as long as they don't leak (discharge). Nicads don't hold charges forever either. As another member mentioned a good charger is worth alot if you going to do robotics unless you want to wait 16hrs. + in between runs. I also use a R/C fast charger with a timer on it. It even plugs into a cigarette lighter. I was lucky that my dad was into radio controlled cars. Too bad I tossed the cars. I wished I saved the motors now.

sam_sam_sam

· CounterRotatingProps

I'm wondering if there's a reference-standard like charging circuit on the 'net --- for what ever reason, there seems to be a paucity of these schematics.

what do you all think of this for Li-Ion charging - it's not mine, nor can I vouch for it, just wondering if it's safe and reasonable as it can be used for several types of batts (according to the author):

http://www.bowdenshobbycircuits.info/page12.htm#lithium.gif

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The last line in the article say this

I would recommend using a large capacitor in place of the battery to test the circuit and verify it shuts off at the correct voltage.

I would also recommend this as well



I have a few of the battery and my problem is not having·a charger·for them this is why I have not use them yet

I am going to try this Charging Circuit·And Do Some Testing·and see what happens and I will let you know

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··Thanks for any··that you may have and all of your time finding them

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Sam