Kwikset Locks and external power

How about spring loaded contacts on the jam side, supplying the voltage, and flush mounted mating contacts on the door edge just above or below the lock. When the doorā€™s closed, thereā€™s power going to the lock. That should work fine since you donā€™t need to power a lock when the door is open.

Alternately, this method could be used to keep the charge of rechargeable batteries topped up.

[Edit] Now that I think about it, probably not a good idea to constantly apply and remove all power to the lock. So I think itā€™s still a feasible idea, but best for keeping batteries topped up

That is how my Jeep works for the tailgate. Contacts allow the rear third brake light to work when the door is closed. Thought about this option too.

So here is another question surrounding the high NiHM battery options: how long do they really last? Both doors are heavy traffic especially the garage door as we go in/out to garage Freezer and fridge multiple times a day. One option I thought about is changing how I handle auto-locking for at least the garage door. Reducing or eliminating the auto lock while people are in the house during a certain time of day/modes. Wifey didnā€™t like this option as she wants the house always locked up and entry only to occur either through manual entry or approved code entry.

So thatā€™s off the table :wink:

Thereā€™s no way to know how long theyā€™re going to last until you try. Itā€™s going to vary for every single lock manufacturer and battery manufacturer and type, and just about every model within a lock manufacturerā€™s lineup

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I like the ideas being presented overall. So just to recap:

  • Use high powered NiHM batteries for best overall performance and potential longevity compared to alkaline.
  • Avoid recharable NiHM batteries if possible unless you can get into a schedule to replace frequently with charged batteries
  • (4) AA batteries is really 6.4v. If hardwiring I will need to take into account there isnā€™t a 6.4v power supply and will need to invest in a 10v power supply with a step down module (probably custom made) to 6.4v at the door lock end of the wire.

My last option may be to invest in Yale locks as I continue to have connectivity drops due to the locks either antenna or power save mode when batteries get below 70%.

Any others?

Oh man... so many variables. If it's a high traffic door: Probably twice as long as Alkaline. if it's a rarely used door: Probably less than Alkaline.

The key is to use high capacity "pre-charged" NiMH cells. Standard NiMH has a fairly high "self discharge" rate which means that even if you keep them in the package and never use them their internal self discharge will cause them to go flat after 6 months. the "pre-charged" NiMH cells will hold their charge for 3+ years. In fact I had a Kwikset 911 (Not zwave, just a basic keypad only) lock before upgrading to the Kwikset 914, and that Kwikset 911 lasted over 3 years on a single charge of NiMH batteries.

The biggest thing with NiMH is understanding the differences in the discharge voltage curve. I've switched every device I own that uses Alkaline batteries toi NiMH so far and have been happy in all cases. The only exception so far is the Kwikset 914's that I just installed a month ago: They still have the alkaline batteries they came with. I figured I might as well use them since there's no sense in throwing away good batteries.

So keep in mind I haven't actually tested the battery performance in these Zwave Kwikset 914 locks yet: But in general in high demand situations NiMH will outlast Alkaline significantly. In low drain situations Alkaline will sometimes last a little longer due to it's almost zero self discharge rate. I find most use cases tend to favor NiMH performance over alkaline. The biggest issue is the battery level reporting will not be accurate because of the difference in discharge voltage curves for the two cells:

I've tested NiMH in a variety of other devices and found that it has a much "flatter" discharge curve than Alkaline and this is what makes reporting battery levels difficult for devices calibrated to an Alkaline discharge curve. An Alkaline battery voltage drops off very quickly. They are rated at 1.5v brand new out of the box: but as soon as you start using them they drop off very fast. NiMH also starts at close to 1.5v when fresh off the charger and almost instantly drops to 1.2v... But then stays there until nearly depleted:

In fact for many use cases (especially high drain) a quality NiMH will actually last longer than an Alkaline and are easier on motors because they hold their voltage under load better.

In my experience most devices that are calibrated to report battery level for Alkaline all offer similar differences in behavior when given NiMH batteries instead: They report an almost immediate drop in battery level to 60% (Because the cells drop to 1.2v so quickly) and then stay there for months. They will then slowly tick down through 30% or so and then drop off a cliff after that. So for each device I want to switch from Alkaline to NiMH I just monitor it and see where that 'cliff" is for that particular device since every device has a different cut-off voltage. A lot of Alkaline devices will cut off at very low voltages such as 0.8v and so for a device like that you may never "see" it hit 20% because the NiMH is completely flat before it drops to 0.8 volts where-as an Alakline still has some life left at 0.8 volts.

In general I find that if you can remember that:

60% Alkaline level = 100% NiMH level
and
20% Alkaline level = 0% NiMH level

This makes it easier to understand the battery levels and not to freak out when you see the battery level drop quickly to 60%. It's just a matter of understanding the calibration. Just aim to change/recharge them around 30% and you'll usually be fine.

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Did someone else say something in another thread about Yale locks? Iā€™m just saying that mine is doing good so far, but I havenā€™t had it that long and my batteries are at 97%. I have no idea how itā€™s going to behave when it gets to 70% or below. I havenā€™t gotten there yet

I think you meant rechargeable lithium. All NiMH are rechargeable. It's best to stick to high capacity "pre-charged" NiMH (and recharge them out of the box anyway) as they have a much lower self discharge rate than "standard" NiMH.

Four (4) alkaline cells would only be 6.0v, and only the instant they are fresh out of the package. They very quickly drop off in voltage and will be under 5 volts the majority of their service life. So a 5.0v power supply should work just fine, it'll just report a battery level of 70-80% or so.

-Jeremy

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Btw: I kind of forgot to address your main point which was the unreliability of the locks reporting at battery levels under 70%. I think that may be a red herring. I have six (6) the Kwikset 914 Zwave locks and I have been having major problems with all of them reporting reliably regardless of the battery level. Most of mine still show 100% and still have issues, and some are down to 20% and aren't any less reliable than the ones at 100%. What I've found is the act of simply removing and replacing the battery pack (Without actually changing the batteries) fixes the reporting issues temporarily every time. So I think you might be having the same problem and you thought it was solved by fresh batteries when it's really solved by just taking away the batteries for a small time and re-inserting them. At least that's been my experience with the 914 Zwave models.

-Jeremy

Yes I have done this too. I believe it is the antennas personally. But could be wrong.

My 914 do experience drop outs with the batteries lower. I use the LADDA from IKEA and they work good for about a month but then I start having issues when the battery reports around 60% . If I put them in something else they run fun until fully depleated, but all my kwikset locks like the batteries charged.

I do have 3 kids so my doors are constantly locking and unlocking.

When talking about Kwikset locks you have to be careful/specific on which zwave module you have/are using - as there are two different ones.

There is a zwave and a zwave plus one.

I have the regular zwave one, and it has been working VERY well. The zwave plus one, however, seems a lot more problematic...

These are the Kwikset 888 which was made specifically for Vivint. They are the ZWave Plus model which just eats up batteries. Alkaline on either door needs batteries replaced every 3-4 weeks. Hoping NiHM batteries get me more time overall. Hoping 2-3 months.

Wow, that's awful. My regular zwave Kwikset are going 9+ months between battery changes.

Well I need to put this in context:

  • Front Door auto locks on average 10-12x/day

  • Front Door auto unlocks on average 5-6x/day

  • Front Door begins to be unreliable when batteries hit 60%. Batteries are still good to use but the lock goes flaky. So about 4-5 weeks of usage for the front door.

  • Garage door auto locks on average 18-20x/day

  • Garage door auto unlocks on average 8-9x/day

  • Garage door also is unreliable when batteries hit 60%. Battery replacement is 2-3 weeks.

So unless someone else is having similar usage patterns I have no idea if this is awful or that I just need to use more powerful and reliable batteries over alkaline.

Thats feels like mine. I wish we had a better way to report battery. Like voltage per cell or something, because then I could have better visibility to when a replacement is required.

That's definitely more cycles per day than my locks get. But I will say mine have been reliable down to 20-30% on the battery level indication.

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:

  1. An alkaline battery at 60% should be around 1.2 volts, times four cells in series gives starting voltage of 4.8v.
  2. That same Alkaline battery at 60% would have close to 300 milliohms of impedance, times 4 gives 1,200 miliohms (1.2 Ohms)
  3. 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:

  1. A NiMH battery at 60% should be around 1.2 volts, times 4 gives starting voltage of 4.8v.
  2. That same NiMH battery at 60% would have close to 60 milliohms of impedance, times 4 gives 240 miliohms (0.24 Ohms)
  3. 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

Now that is some analytics!

Yes doors can auto lock because you need to manually unlock to get out. Appreciate the deep thought and analysis on my problem. :grin:

Not exactly the same you guys want to do here but I got some inspiration from this thread and recently MacGyvered a solution to cable an August Keypad that kept dying with drained batteries due to the cold Canadian weather... May give you some useful ideas:

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