Based on this description of your use case I'd almost be willing to put money on the NiMH batteries working better for you. Plus they are rechargeable so you won't be buying so many replacement batteries anymore. One thing though: How are your locks and unlock counts different? Unless I'm missing something: How can a lock be locked more often than it's unlocked? Or are some of the unlocks done manually rather than electrically?
NiMH tends to perform better over Alkaline for heavy loads because it has 1/5 the internal resistance (impedance). Alkaline is around 150 milliohms per cell vs 30 for NiMH. The internal resistance increases as the cell discharges which is probably why you're having so many issues at 60% charge (The impedence increases as the cell discharges, doubling at 50%) and it also increases as cells are connected in series. Since these locks use 4 batteries in series that's 600 milliohms and that doubles to 1,200 milliohms at 50% charge.
I haven't tested the load these locks place on the batteries when they lock/unlock: But it's probably a decent amount of current especially if the deadbolt encounters any physical resistance at all as it slides in/out. (Have you checked that the mechanism slides very easily? Even the slightest resistance will make a huge difference on battery life.)
Just because I'm curious I did some quick math to estimate the current these locks might pull. Kwikset claims 1 year of battery life at 10 cycles per day. (I'm assuming that's 5 locks, 5 unlocks for 10 total engagements of the motor). Alkaline batteries offer about 2000 mAh of capacity. So that means one set of batteries should be able to engage the motor 3,650 times. 2,000 mAh / 3,650 engagements = 0.55 mAh per engagement.
Now that we know each engagement of the motor eats about 0.55 mAh of energy, to calculate the "instantaneous current" we need to know how long the motor runs. If we assume it operates for about one second we would multiply 0.55 by 3600 because the 0.55 is expressed in "milliamp hours" and there are 3600 seconds in an hour.
So 0.55 times 3600 gives 1,980 milliamps or just shy of 2 amps. That sounds a bit high for such a small motor but it also needs to be fairly high torque to do it's job properly so it's not a completely unreasonable ballpark estimate. (I still plan to hook my meter up to one of the locks to measure the actual current draw to be sure)
So we know:
- An alkaline battery at 60% should be around 1.2 volts, times four cells in series gives starting voltage of 4.8v.
- That same Alkaline battery at 60% would have close to 300 milliohms of impedance, times 4 gives 1,200 miliohms (1.2 Ohms)
- Voltage drop is given by: V = IR : So V = 2.0 amps times 1.2 Ohms = 2.4 volts!!!
That means that after the Alkalines drop to around 60% that anytime the motor is engaged the battery voltage is being pulled down quite significantly. Probably enough to cause the radio to have trouble transmitting.
If we re-run the same math for NiMH:
- A NiMH battery at 60% should be around 1.2 volts, times 4 gives starting voltage of 4.8v.
- That same NiMH battery at 60% would have close to 60 milliohms of impedance, times 4 gives 240 miliohms (0.24 Ohms)
- Voltage drop is given by: V = IR : So V = 2.0 amps times 0.24 Ohms = 0.48 volts.
So, at 60% state of charge, the Alkalines would drop from 4.8v down to 2.4v when the motor is engaged, and the NiMH would drop from 4.8v down to 4.32v. The Alkaline voltage drops by half which is certainly enough to make the lock behave unreliably.
Obviously this is based on a lot of assumptions. I could be way off on the motor power requirements. But regardless you can quickly see how the NiMH would hold up better under high current loads if those motors really are pulling that kind of power and being used very frequently. Even at only 1 amp of current you would see similar differences between the chemistries performance.
-Jeremy
