Hey Folks,

I'm looking to build a charger to charge a 14.4v 3800mAh pack. Well, I really want to charge 4 of these up. But, from what I can tell, charging NiMH packs in parallel is bad.. So, I though I could build 4 charging circuits and wire them in parallel using Diodes. I've read the Data sheet on the MAX712, and It looks like what I need. I want to be able to charge these packs at around 1.25 to 1.5Amps. Source voltage will be a 18v power supply with up to 8amps output. The circuit needs to be able to handle the load while charging.. The load can peak at around 1.7Amps. I thought about using larger batteries such as D's or F's.. However, charging multiple packs at the same time would yield a faster charge.. and the most mAh I can get with other cell's is either 10000mAh with D's or 14000mAh with the F's. 4 3800mAh packs gets me longer runtime. Any help would be appreciated!

Thanks, Connor

 

The datasheet for the MAX712 shows each cell's voltage exceeding 1.5V. The datasheet says that the input voltage must be at least 1.5V higher than the max battery voltage. You are charging 12 cells so you need an input voltage at least 20.1V.

The IC has only a single input to detect when the battery is fully charged. Then you can't add more batteries with or without diodes.

__________________
Uncle $crooge

 

The power supply I'm using is a universal power notebook adapter. Output is rated for 15-24v up to 8.0A 120W Max. Current plug I'm using is around 18.5v or so.. The battery packs are 14.4v packs. So, I thought I would be good with 18v??

I was thinking about using 4 MAX712's 1 for each pack, then put diodes on the output side of the circuit before wiring them to the load, Thus, each MAX712 is responsible for it's own pack.. and the load is shared across all 4 packs/chargers.

Thanks, Connor

 

If the input voltage is not at least 21.1V then your batteries will not fully charge.
Don't use diodes at the output of each MAX712. Each one charges its own battery. They cannot share a load because each one must detect that its battery is fully charged.

__________________
Uncle $crooge

 

This is the part I don't understand. The MAX712 says it can charge a battery while supplying a load. Looking at the found @ http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666 You couldn't put a diodes right before the load so that the charging circuits were isolated as far as the charging side?

Thanks, Connor

 

I want to use this ic for a project im building, but after reading the datasheet i found the part about the number of cells programed a little confusing. it says: "The internal ADC’s input voltage range is limited to between 1.4V an 1.9V, and is equal to the voltage across the battery divided by the number of
cells programmed". does this mean i cant use it with a single 8.4V 200mAh NiMH battery? Would using 8x 1.2V 1400 mAh (AA) batterys be better? And finaly if i do use the AA batterys in series will the current/h rating be additive (iow 8x 1400mAh = 11,2 Ah)?

 

The Ah rating of a battery is for its current. Cells in series all have the same current as one cell. So the Ah rating of a battery with one cell is the same Ah rating of a series srting of many cells.

I didn't read the entire datasheet. You must program how many cells you have and I think a resistor determines the charging current.

__________________
Uncle $crooge