DS2715 NI-MH Charge Controller Help

Question

I am attempting to design a system that charges a 6 cell 1800 mAh NIMH R/C car battery wirelessly with the DS2715 charge controller by Maxim. I am trying to modify their linear charging design for a 6 cell battery. In the data sheet they specify the circuit for charging a 3 cell pack (in series) linearly (with a 6 V supply), and I have built the circuit accordingly.

The only info in the data sheet regarding changing the cell count is the voltage divider on the Vbat pin for detecting a single cell's voltage. I assume that for charging 6 cells I would double the supply voltage for 12 volts? I know that the IRF can handle this change but am unsure about the other components and do not want to damage the IC.

When I supply the circuit with 6 Volts and use the original resistive divider for 3 cells, the LED is off (as it is supposed to be with no battery connected). But, when I use the 6 cell resistive divider and supply 6 V the LED flashes at 4 Hz indicating that there was a fault. I am assuming this is because the circuit really does need 12 V. I have a LED that is suited for 12 V on the way but wanted to see if anyone has used this IC for a similar design and could offer some insight.

I have contacted Maxim for support but they have not gotten back with me yet. Thanks in advance.

Part link: https://www.maximintegrated.com/en/products/power/battery-management/DS2715.html

Datasheet: https://datasheets.maximintegrated.com/en/ds/DS2715.pdf

Answer 1

Based on the table "RECOMMENDED DC OPERATING CONDITIONS" on page 2 of the datasheet you provided, there should not be any problem with connecting 12 V to the IC, as the maximum recommended voltage for Vdd is 16.5 V. The IRF has a blocking voltage of -60 V (see 1), so it could handle the 12 V as you already mentioned.

The DS2715 has a presence detection that is based on the voltage on the Vbatt pin. On page 6 of the datasheet under section "PRESENCE" you can find one of the conditions needed for the detection of a connected battery pack is Vbatt > Vopen. Vopen is fixed to about 1.65 V (page 3). Without a connected battery pack the voltage Vbatt should be higher than this threshold to avoid a false detection. In your case the voltage divider was designed for a 6-cell battery pack, where i assume that the voltage on Vbatt is 1/6 of the supple voltage (6 V), therefore the voltage is lower than Vopen. If you apply 12 V to the circuit, the voltage Vbatt will increase to 2 V (> Vopen) and the IC should not detect a connected battery pack.