# MAX712/713 NiMH battery charge expectations

#### P.feldman

Joined Jun 18, 2022
6
Hello, First post here, but have visited many times in the past.

Working with MAX712/713 to charge 1 NiMH Panasonic HHR-75AAAB cell (750mAh). I am able to charge the battery with the base circuit provided, with some modification.

1. Manual for the charge controller indicates a minimum charge block output of 6V. Most phone chargers I have are rated at 5V, 9V and 12V DC. When the circuit is plugged into this charger, the output is just under 5V DC (cheapo DMM, not oscilloscope) and the circuit charges the battery.
How do I interpret this requirement correctly? Do i need a charge block that will only output above 6V?

The given conditions make it impossible to calculate R1. I have tried changing R1 resistance until i get at least 5mA at V+, but the resistance needs to be dropped to nearly a short circuit to get this result. I suspect things are functioning as intended and I need to bump up the input voltage. Currently, am charging with ~2.5mA to V+.

2. I am trying to achieve the 450mA of current to the battery (as per battery fast charge spec), and am able to achieve this with RSense=~0.7 Ohm. This is derived from manual formula. Concern is that resistor Q1 gets hot to the touch after several minutes of operating (~70-80C, temp does not run away), I have it at about 250mA charge now and the heat does not build in the transistor at all. The transistor is rated to 165C, but It doesn't mean that it should dissipate that power. Maybe this concern is a result of the power requirements.

Phil.

#### ci139

Joined Jul 11, 2016
1,898
i've found NiMH safe to charge at 180mA - so their T won't rise too high . . . i speculate your 450mA option may limit the number of replies - not because such might be impossible - but because it requires specific and detailed know how

#### P.feldman

Joined Jun 18, 2022
6
i've found NiMH safe to charge at 180mA - so their T won't rise too high . . . i speculate your 450mA option may limit the number of replies - not because such might be impossible - but because it requires specific and detailed know how
Thats good to know.

I am guessing you are referring to the temperature of the battery won't rise too high? Or did you mean the transistor?

I'm okay with the lower charge rate, and I'm happy things are working, im just not sure what to expect with failing the criteria for the minimum input voltage and minimum current to V+.

#### P.feldman

Joined Jun 18, 2022
6
It seems like the manual has some inconsistencies, they explain the purpose of having minimum 1.5V above maximum cell voltage, but they do not repeat or explain the reasoning behind the 6V minimum. Not sure if it is something internal to the controller or an error in the documentation. I will see if it fails to terminate fast charge.

#### ci139

Joined Jul 11, 2016
1,898
they do not repeat or explain the reasoning behind the 6V minimum.
This is a question to manufacturer or their support forum.
- as they partially hint - it may be required to ensure enough power for the controller's supply (i haven't red) --and/or-- for the consistent charging current-/voltage - levels

(i may be a bit frustrated right now because of the local warm weather)

#### P.feldman

Joined Jun 18, 2022
6
Wanted to follow up on my findings:

The MAX712/713 in fact needs more than 5V (recommended 6V+) from the DC supply. Otherwise you break the minimum 5mA to the V+ pin, meaning your fast charge characteristics may be unpredictable. I got close to 5mA, but can't risk the unpredictable.

Unfortunately most, if not all phone charge blocks i find are 5V DC out. I know fast chargers exist for 9V, but its not practical in this situation to implement a solution for forcing that output.

This makes this chip HEAVILY dependent on the power supply adapter. I was hoping to provide a solution that allows you to plug in via USB to most blocks.

There are many alternative chips such as LTC4010,4011,4060 that I am looking at now that don't fail this criteria.

The MAX712/713 advantages are high charge rates with large cell count capability when compared to other options on the market (considering external components required to assemble).

#### ci139

Joined Jul 11, 2016
1,898
many Li-ion-s are protected by on-board module (limiting the I/O current) +the cell-phones may have their own charge controllers

you can only benefit from your idea on the bare batteries . . . just a reminder

#### Ya’akov

Joined Jan 27, 2019
8,973
If you want a 5V supply, you could use a boost converter to get the voltage up to 6V at the cost of some current.

#### ci139

Joined Jul 11, 2016
1,898

I don't quite get your confusion (fast glanced the d/s) - it shows there is a +5V shunt regulator on board - so if you provide two supplies one for powering the chip and another for charging the batteries you need not worry about the supply specs for the V+ pin . . . the Fig.2 shows a CC feed !!!

however - what sounds suspicious is using the same profile for the NiCd and NiMH cells - the behaviour of these types is a way different at the end of charge also at the beginning of charging - the variation of NiMH battery types are also quite wide . . . there are of course options available to configure these charge controllers in several ways . . .

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#### P.feldman

Joined Jun 18, 2022
6
If you want a 5V supply, you could use a boost converter to get the voltage up to 6V at the cost of some current.
This is the kind of answer I was looking for! It will likely work, only thing is I need to account for extra circuitry and cooling.

#### P.feldman

Joined Jun 18, 2022
6

I don't quite get your confusion (fast glanced the d/s) - it shows there is a +5V shunt regulator on board - so if you provide two supplies one for powering the chip and another for charging the batteries you need not worry about the supply specs for the V+ pin . . . the Fig.2 shows a CC feed !!!

however - what sounds suspicious is using the same profile for the NiCd and NiMH cells - the behaviour of these types is a way different at the end of charge also at the beginning of charging - the variation of NiMH battery types are also quite wide . . . there are of course options available to configure these charge controllers in several ways . . .
My circuit does work right now, the problem is I am not meeting "critical" requirements outlined in the data sheet because of the power supply. Ultimately, the warning is unpredictable fast-charge termination. This is not a risk i can take with this application.

I have found an alternative in my toolbox that actually works phenomenally well in comparison! The LTC 4060 fits the bill much better, with less hardware.

I have also done some studies on power dissipation and the heat I am seeing on the external transistor falls inline with what the power dissipation calculations show. Simply a rookie mistake on my end. improving the cooling situation will allow me to push much higher current into the battery.

Thanks for all the replies guys!