# What is a good resistance value for voltage divider to measure LiPo voltage with Arduino

(Full disclosure: I'm a newbie.)

I'm trying to measure my 4S lipo's cell voltages using an Arduino Mega. I know that I should use a voltage divider (actually, several of them) and I think I understand how it works. I also know about the voltage difference relative to GND would increase by up to 4.2V for each cell, and at 4th cell, it can be over 16V so I should be safely dividing it down to max 5V. However, I'm confused with what resistance values should I use for the divider(s). Wouldn't a low resistance value mean effectively shorting the lipo? Would a too high resistance value cause incorrect readings? In this context, what is a "low" resistance value and what is a "high" resistance value? Or are all resistors already way above that limit and I can use them safely? I've got all types of resistors from 47R to 1M, but I don't know what to use.

• Make it small enough to not be influenced by the measurements devices input impedance, make it large enough to not draw too much current. Aug 17, 2015 at 14:21
• @PlasmaHH how do I determine that? e.g. is 1K okay, or is 470K okay? Aug 17, 2015 at 14:22
• Without looking up the data sheet - if you don't mind needing to disconnect it when not in use - ie on the load side of the on/off switch or break plug, then a total of 10k-20k range will be very safe. If Full scale is say 20V then you want a 4:1 divider to 5V so 3R:R so say 15k Bat to ARduino and 4k& Arduino to ground. You could use a pot for part of that OR adjust in software or trim with parallel resistors. || Odds are you can go to 50k or even 100k and be not too inaccurate but lower is better. Arduino uP will have an ADC Rinmax spec somewhere. At 20 V and 10 =bits = 1024:1 divide you... Aug 17, 2015 at 16:04
• ... get about 20 mV per bit so accuracy of about 0.1V allows 1+ bits of error. Error is caused both by Verror = Rin x (Ibias + Ioffset) PLUS by time constant issues where input c takes time to charge and if R is too large the cap will not be charged enough by next ADC samople time. Aug 17, 2015 at 16:07
• ... You also can use an opamp buffer that can tolerate very high Rin. At say 1M the drain usually does not matter over usual time periods. || (Past bed time in NZ . 4:15am and I have to be up at 8:15 so ... TTFN). Aug 17, 2015 at 16:16