| Back to Summary |

From: "GrantM."
Date: Tue Apr 20, 2004 3:54 pm
Subject: Re: [beam] 25 minute runtime, 5 hour charge time
http://groups.yahoo.com/group/beam/message/42795

Hi Wolf,

At 11:00 AM 4/20/2004, you wrote:
Hello group,

I am still working on the battery charge circuit. The batteries are
two 4.8V 280mAh Ni-Cad wired in parallel, and the circuit is the
battery charger from the BEP documentation [1,2].

Originally, the problem was that the DC charger had an output of ~5V
and would not fully charge the batteries. I have replaced the charger
with an 8V rated / 12.9V actual adapter.

Now, the problem is that the indicator LED does not turn off.

Given the rather flat charge curve of charging Ni-Cads this is not unexpected.

 
Additionally, I am concerned about over charging the Ni-Cads. I get
around a twenty-five minute runtime on a five-hour charge.

Generally if you keep the charge current below 1/10C (say 56mA) the batteries can dissipate the extra energy as heat and not be damaged.

Given the
circuit draws 32.33 mA on average, the batteries are not fully
charging.

If the charger did drop down to 8V during charging the actual charge current would be less than 20mA, charge for 5 hours is 100mAH ideally. You draw about 13.9mAH giving a battery efficiency of 70%, IIRC if you charge at 1C Ni-Cad charge efficiency is around 80% to 90%, but you would be charging at a significantly lower current.


I have the following questions:
1) What should I change on the BEP-25cChg to facilitate the ~12V
adapter and the 4.8V batteries?

If you change R1 to 430 ohms this should bring the charge current close to 1/10 C (28mA). The current passing through R2 should remain fairly constant due to the voltage drop of the LED.

oh wait you said two batteries in parallel, just change R1 to 160 ohms for a charge current of 43mA.

One thing to try is measuring the voltage output of the adaptor while charging the batteries, the load may cause the output voltage to drop to the specified 8V. In which case R1 = 36ohms.

2) Is there an alternative charger circuit that I should use? I am
using perf board for this circuit, and am not limited to the BEP
board.

That all depends on how complex you want to get, the BEP charger is a fairly bare bones type charger. Quite a few chargers for cordless drill's, RC cars ect. just use the current limit of the wall wart for regulation! talk about crude.

I have read that Ni-cad batteries charge most efficiently with a charge current of .5C to 2C (C being the mAH of the battery). That being said a simple charger can be made out of a LM317 to keep a constant current charge on the battery but you would still need to manually disconnect this when the battery is full.

So how to determine if the battery is full?

The simplest method is to measure the charge current and figure out how much time you should leave the batteries on the charger.

One property on charging Ni-cads is that they will start to dissipate heat when they are fully charged. If you monitor the temperature wait until they are about 10 degrees above ambient temperature and get the charger to shut off,

The voltage of the Ni-cads doesn't vary that much during a charge ( roughly 0.15V per cell ) but there is a bit of a "hump" in battery voltage when the cell is fully charged. This "hump" (10mV per cell) is what peak chargers use to determine if the battery is full.

A good charger would monitor all three and keep track of the capacity of the battery as well.


3) What voltage do the Solarbotics BattR12's normally charge to?
Currently, they are charging to ~5.6V.

Its a 4 cell battery with a nominal voltage of 1.2V per cell, the operating voltage is 4.8V. However when charging the voltage can be up to 1.5V per cell or higher depending on the current that you are charging at. 5.6V across the batteries while charging is not unreasonable, under a load however they should drop close to 4.8V

 
Any assistance is, as always, most appreciated.

J Wolfgang Goerlich


Related Links:

1. Solarbotics BattR12, 4.8V 280mAh rechargeable Ni-Cad
http://www.solarbotics.com/products/index.php?search_id=209

2. Solarbotics BEP-25cChg
http://downloads.solarbotics.net/PDF/Bicore_Experimenters_PCB/BEP-
25cChg.pdf


Don't even get me started on NiMH or LiPoly...

GrantM.

From: "J Wolfgang Goerlich"
Date: Wed Apr 21, 2004 4:41 am
Subject: Re: 25 minute runtime, 5 hour charge time
http://groups.yahoo.com/group/beam/message/42807

Hello Grant,

The answer along with the mathematics. You are a gentleman and a scholar.

The voltage output of the power adapter is 11.979V while charging. At a 160 Ohm value for R1, I can charge the batteries up to 5.706V in six hours. At 200 Ohm, I can charge the batteries up to 5.702V in the same time period. This has given me the longest runtime thus far: a shade over five hours.

However, my LED still is shining bright though the charge appears to be complete. The LED is a green Solarbotics Tiny LED. This is the last hurdle, I think.

If this hurdle can be cleared with adjusting the resistor value or adding a part or two, then I would like to clear it. Otherwise, I think that I have learned enough about batteries and will simply set a timer when I charge my Turbot.

Many thanks,

J Wolfgang Goerlich