help with modified minty boost project

[lokeycmos]

so im building the minty boost project here: (1st schematic on the page)

http://www.ladyada.net/make/mintyboost/parts.html

i want to put it in a project box with rechargeable battery pack and a jack to charge it from a wall wart. the IC has a max volt input of 5v. for batteries i have 12 duracell NiMH AA 2650mAH. not sure if i should go for 4.8v@7.95AH or 3.6v@10.6AH. the watt hours are the same. the trickle charging circuit im thinking of using is here(circuit #3): **broken link removed**

i included a hand drawing of what im thinking. i would like to hear feedback if this will work or if i need to change anything. i do need some help finding the correct voltage and amperage for the wall wart to use as well as resistor values to use in the trickle charger.

 

[cowboybob]

not sure if i should go for 4.8v@7.95AH or 3.6v@10.6AH

The datasheet for the LT1302-5 indicates that that chip efficiencies go up with higher Vin values. Not a great deal, on the order of 4 to 6 %.

The datasheet also, and perhaps, more importantly depending on your usage plans, indicate that the "Maximum Output Power" in Boost Mode goes from approximately 6W to 8W with an increase in Vin from 3.5VDC to 4.8VDC.

The datasheet also notes that Vin can be as high as 8VDC, but that is a MAX value and probably should be avoided.

All of which is to say that the closer Vin is to the resulting Vout, the more efficiently the chip operates.

So it would appear that, all things considered, the 4.8VDC battery option would be preferable.

The trickle charger page link failed, so that issue is not resolved. I will add, though, that NiMH batteries are a lot more forgiving that Li types when it comes to charging rates.

 

[lokeycmos]

here is the link to the trickle charger. its circuit #3 towads the bottom of the page.

http://talkingelectronics.com/projects/ChargingNiMH/ChargingNiMH.html

 

[cowboybob]

OK.

That'll work fine, except, of course, swap out the 12VDC wall wart (WW) for one with an output slightly greater than your selected battery set's nominal voltage. For NiMHs I'd say, 15%, or so.

For instance, with a 4.8 VDC battery set, use a WW as close to 5.5 VDC as possible. They'll charge slowly, but safely.

And follow the advice given in the example you cite regarding monitoring the current and voltage levels to assure yourself you're not frying the battery(s).

 

[lokeycmos]

OK.

That'll work fine, except, of course, swap out the 12VDC wall wart (WW) for one with an output slightly greater than your selected battery set's nominal voltage. For NiMHs I'd say, 15%, or so.

For instance, with a 4.8 VDC battery set, use a WW as close to 5.5 VDC as possible. They'll charge slowly, but safely.

And follow the advice given in the example you cite regarding monitoring the current and voltage levels to assure yourself you're not frying the battery(s).

thanks for the reply. so my NiMH battery pack will be 4.8v @ 7.95AH. i have a huge box of more than 50 wall warts to choose from. for these specs, what wallwart(V and A) and resistor values would you recommend for this trickle charging circuit? i want to trickle charge for 2 reasons: 1 i can set it and forget it for days at a time, and 2, its healthier to trickle charge the batteries. thank you!

 

[lokeycmos]

can anyone help me calculate the best resistor values and wall wart ratings for this trickle charger circuit? circuit number 3:

http://talkingelectronics.com/projects/ChargingNiMH/ChargingNiMH.html

my battery pack is 4.8v @ 7.95AH thank you in advance!

 

[ronv]

Use a 10 or 12 volt wall wort and change the resistor to 100 ohms. Then adjust the current to 200 ma. (.025C). The transistor may get pretty toasty so keep the leads short to the board.

 

[lokeycmos]

Use a 10 or 12 volt wall wort and change the resistor to 100 ohms. Then adjust the current to 200 ma. (.025C). The transistor may get pretty toasty so keep the leads short to the board.

thanks. just to clarify, change the 470 to 100 ohms and adjust the pot to 200ma?

 

[ronv]

Right you are.

 

[lokeycmos]

can i substitute a TIP31A for the BC338?

 

[ronv]

Should be okay.

 

[lokeycmos]

i know that this is a current regulator, but is it safe to have 18v on the output even though the current will be limited to a safe level of say 200ma? in a current regulator it keeps the current steady regardless of voltage. so will my batteries be ok if it has a high voltage but safe low level charging current? thanks.

 

[cowboybob]

will my batteries be ok if it has a high voltage but safe low level charging current

Should be. Although even less quiescent current would better.

Best to monitor battery temperature after full re-charge for a while.

 

[ronv]

Yes, I think so. With no load (battery) the regulators don't work and the power supply goes to 18 volts.

 

[lokeycmos]

Should be. Although even less quiescent current would better.

Best to monitor battery temperature after full re-charge for a while.

what is quiescent current? ty

 

[ronv]

The batteries don't like to be charged continuously at more than .033C or 3.3% of capacity. So in your case I picked 200 ma
(200/7950=.025). Having said that I'm not sure what your pack looks like. Is it 4 in series and 4 sets in parallel?

 

[lokeycmos]

4 in series in parallel with 4 in series in parallel with 4 in series

 

[ronv]

Should be okay, but as Bob says make sure they a not hot at the end of charge.

 

[cowboybob]

what is quiescent current? ty

That current that continues to flow in a circuit when the circuit's function is off (think of a TV. You click it off, but it's still drawing current to feed the remote pick-up).

In your case, the natural resistance of the batteries, even when fully charged, still represents a load (small, to be sure) but a load nonetheless. That little bit of current is quiescent current.

There may not be any in your circuit. That's why I suggested checking battery temps. If they stay warm, you've got quiescent current. That being the case, best to remove the batteries from the charger after the charge is complete.

"Trickle chargers" do just that: continually exhibit quiescent currents.

Nice bread board work, by the way. Very neat and tidy (at least compared to mine).