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Battery Current draw when operating

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  • Battery Current draw when operating

    First and apology. I inadvertently posted this earlier in the Stairlifts for Sale section. I'll try to be more careful next time.

    .Hi All,

    I'm new to this forum and a bit of background in under my 'activity' tab.

    I'm experimenting with two 12volt 7amp batteries from an older stairlift. Several videos on the internet indicated that when these (sealed lead acid) batteries age, the lead plates get corroded but can be rejuvenated. I've tried some techniques to rejuvenate these batteries and results appear quite good so far. Both batteries after recharging several times, while carefully keeping the distilled water above the lead plates during recharge, show 12.3 and 12.7 volts.

    The next test is to determine if the batteries will produce the current ( amperage) needed to drive the stairlift. I couldn't find this info on a internet search. I'm sure the amount of amps is dependent on the angle the stairlift rail and the weight of the person on the lift. But I'm wondering if anyone can give my a general idea of what amps the batteries have to produce while the lift is going up the rail. Going down the rail should be a lot less.

    My technical inquisitiveness causes me to do this. :-)

    I'd appreciate any comments.

  • #2
    Stairlifts are type tested and accredited to BS safety and also EU Harmonised EN81 standards therefore if batteries are new the lift would be capable of 10 full journeys (1 journey = Up+Down) in a 24 hour period with a new set of batteries and fully charged. Batteries should last for 3 years for the design to comply under normal charging conditions.

    As you say angle and current will dictate the current draw times of availability, as will servicing of the lift and end users weight and rail length and so on.
    There are many variables; however the preceding at full load and maximum angle still remains at 10 full journeys. You mentioned playing around with the batteries and voltage rates; those rates are low for a battery not under load and if that is fully charged, even lower if that is the charge voltage being applied. With a low voltage you will not have enough electromotive force to supply the required running currents. All tests would need to be conducted with new batteries as a reference to the original design and at full rated load, angle, and maximum lenght of rail which I doubt you will be able to recreate.

    To answer your question of longtivity, I assume your batteries are 7Ah therefore you would be able to measure your maximum current draw from either the lift specification or through directly measuring draw using a full and new set of batteries and simply using Ohms Law to calculate design.

    My experience is that you will not restore your existing batteries even with high voltage de-sulfation method and will remain to suffer. If your batteries are aged they they won’t store a full charge through natural detereation and if they have been left to discharge below 5volts they will be damaged internally anyway.
    Last edited by /robbrownstairlift/; 13-03-2019, 05:08 PM.


    • #3
      How u topping the distilled water up when stairlift batteries are generally of the gel type with NO fluid.......


      • #4
        Hi Rob and Clive, Thanks for your comments. They are much appreciated.

        The batteries I'm working with are from a 5 year old Acorn 120 lift. Genesis NP7-12FR 12volt, 7.0 AH..

        In response to Clive's question: When I removed the top of the sealed battery ( as described in several internet videos), one sees the 6 rubber like removable tops of the six 2+ volt cells which make up the 12+ volt total. Removing the rubber like tops, peering into the top of the cells, some had a liquid covering the plates, and some did not. I assumed the liquid was probably water or weak sulfuric acid. Two additional factors gave me confidence that these batteries had liquid electrolytics: (1) the battery case had a warning indicating sulfuric acid was a component part of the battery, and (2). the 16 page Genesis Application Manual for the NP and NPX batteries ( downloaded from the internet) had these words in a chart " All Genesis® NP batteries utilize an electrolyte suspension system consisting of a high porosity, glass fiber material which in conjunction with plates, totally absorb and contain the electrolyte. No silica gels or any other contaminants are used." I concluded from all this that the cells used distilled water. I have to admit however, I don't have a 100% record of coming to the right conclusion! But the fact that the batteries have reached a bit over the 12 volt level gives me a bit of confidence that I'm on the right track..

        In response to Rob: Your comments regarding how lift batteries are tested through 10 journeys in 24 hours was quite informative and helpful for my thinking process. My past experience with lead acid auto batteries gives me doubts also that it is difficult, if not impossible, to restore an older/hard used battery to 100 % of it's initial ratings. My curiosity and spare time is leading me to experiment with these sealed batteries. I don't have the lift I purchased installed on stairs yet, otherwise I'd simply measure the battery amps when the lift was running up and down the stairs. I was hoping someone could give me a general idea for what that range might be. I'm going to see if I can get any information on the specifications of the motors used in lifts. That could perhaps give me an idea of an upper limit of current for the motor design horsepower. 0

        Two graphs in the Genesis Application Manual for the NP and NPX batteries tweeks my curiosity even more about the capabilities of the batteries used in lifts. . It's Fig.1 shows battery current discharge vs time, and implies ( to me) the NP7 can discharge 20 amps for a couple of minutes. Fig.3 shows discharge time vs battery terminal voltage as a function of current drain. It also shows the NP7 at a drain rate of 3 CA ( I'm thinking that implies 3x7 or 21 amps) as having a terminal voltage of 11.3 volts for a minute, and dropping to 10 volts at 5.5 minutes. That's a surprising capability to me.

        Whoops, didn't mean to get so long winded. Thanks again for your comments.



        • #5
          I would be very cautious as I don’t think there is a full understanding here. It is clear from the battery spec provided that these are SLA batteries. Which are the little rubber things you refer to IE the gassing or venting valve to prevent spillage of the Acid electrolyte. The batteries were also originally classed as fire retardant. A basic understanding of Ohms Law (I=VxR) answers all questions. For example if your lift is pulling 1 amps (more likely 1.5 - 2 amps) per journey and each journey takes 1 minute (Journey is start of travel to top of the rail and back to the bottom) and batteries in your case are capable of delivering 7 amp in 1 hour then your lift should run for 60 minutes * 7 amps max = 420 mins which equates to 7 Hours use from that small little battery when new and fully charged. I have concerns over how the batteries are being charged & tested also no longer sealed so may spill acid or give off fumes for ignition if in a confined space so will not comment further on specs and design of charging circuits and their watchdog monitoring systems. I would advise your batteries will not recover and that disposal must be via controlled waste as they are now in a hazardous state owing to being opened up and no longer sealed.


          • #6

            Your absolutely right about the V=IR equation, a lot can be done with it. But there are numerous factors that have to be taken into account when applying it or the result is completely wrong. For example, the resistance of an incandescent light bulb differs when the bulb is energized or not. The bulb' s resistance changes as it gets heats up so the initial current is not the 'steady state' ON current.. Another example relates to my current interests: current drawn by a stairlift motor is not the current calculated by I= V/R using the resistance of a motor when not energized, it is much less. As a motor begins rotating a 'back EMF' builds up and causes the running current to be much less. In this case the R used in the I=V/ R would have to the the 'virtual' R ( a result of the back EMF) of the motor when running.

            I suspect from the Genesis NP7 battery manual's Fig.3 the stairlift's electrical designers make use of the batteries capability to provide 21 amps for up to 3 minutes before the battery terminals drop to 11 volts. I believe the Acorn Superglide 120 batteries I'm working on draw more that 1.5 to 2 amps. How much however is the question I'm trying to get and answer to.

            I appreciate your concerns about how the batteries are being charged, the fumes during charging ( hydrogen gas .very explosive ), the sulfuric acid, and disposal..Thankyou for bringing up your comments on safety concerns.. I am aware of these issues and have taken them into account. in what I am doing. I certainly would not recommend anyone trying what I'm doing without a solid electrical and mechanical background, or under the supervision of a qualifid technical person.


            • #7
              Rob and All,

              Can anyone provide me the fuse rating ( volts, amperage ) of the fuse that in in the positive lead of the batteries pf the Acorn Superglide 120? That would give me an idea of the maximum current the electrical system's designers felt was necessary for the batteries to deliver to protect the motor and printed circuit board electronics.


              • #8
                I found this page on the internet showing motors for stairlifts by Parvalux along with their power rating in watts yesterday. A Watt (W) is equal to Voltage (V) times current( I ) and written in formula form : W=VI or I = W/V. The stairlift motors Parvalux shows in its sales brochure (https://www.parvalux.com/application...are/stairlifts) have high side power ratings (W) from 345 to 583 watts. Using the I = W/V and using 24 volts as the lift voltage, the max current (amperage) for these motors should run from 14.3 amps to 24.3 amps.

                Assuming the electrical designers built in a 50 % overload/safety factor in design, the amperage draw would be from about 7 amps to 12 amps. They seem to be reasonable numbers to me. Knowing the voltage and amperage rating of the fuse of the Acorn lift would allow one to get closer to the actual running amps. By the way, one fellow on the internet claims to have built a circuit using a 12 v 7 amp battery that operated at 100 amps for 1 minute.

                One other note: a Lemac 24 volt motor reportedly from an Acorn stairlift on the internet stated the motor was 375 watts at 53 rpm. With that info I'd think the Acorn lifts may run somewhat below 7 amps and have a fuse rated about 10 amps or more. As I noted in a previous post, I'd expect the starting amps to be higher ( until the motor gets to its normal running speed) so the higher fuse rating would account for that..

                Slowly but surely zeroing in on getting to understand how the lift motor and batteries work.

                Comments appreciated.


                • #9
                  Drawing 100 Amps from a 7ah is possible, in an ideal case a fully charged 7Ah battery would deliver 100amps before being depleted for 70 seconds. IE Time = Capacity/current drawn T=7000/100 = 70Seconds.

                  The key in calculating battery life is to use the watts you know for the calculation of the amps at the battery voltage. You are way out on fuse ratings as the opening current you refer would cause nuisance tripping, However, As above a fuse opening time is also related to the current it receives and time period it passes, rupture is normally many times greater than the actual rating it is given and denoted by Time lag, slo blow, fast acting etc for example the circuit may employ a 15 or 20amp fuse but a thermal overal overload rated at 5 amps. Rating is all design dependant.

                  With respect to the differing motors you refer all of this is taught in an academic qualification - Stairlift Technology, Output RPM is worked out from torque required and PCD of Rack and pinion. Your opening calculation law is also used widely in calculating power ratings. As mentioned earlier the actual running current is much less than you think, more like as I have posted, less than a couple of Amps, start up current is very brief. As above I suggested running times and 10 journeys. A simple calculation of working out the average watts consumed over 20 starts (10 full journeys’) and watts used with the lift not charging and in standby for the remaining duration of the 24 hours would be a simpler calculation to confirm power required against power used. Design factors of safety are also fully explained in the course.
                  Last edited by /robbrownstairlift/; 19-03-2019, 12:25 PM.


                  • #10
                    rob and all,

                    Thank you for all your comments, suggestions, recommendations and concerns. They have all been very helpful in me thinking about the power requirements and in how stairlifts operate. My latest inquiry was to Acorn and I asked if they could tell me the amp rating of the fuses used in the Acorn stairlifts. The gentleman I talked to said he'd need to contact the technical folks, which he did while I waited on line. He came back shortly saying there is a 20 amp on the motherboard board and a 15 amp in the line from the battery. That helps satisfy my curiosity at the moment.

                    I was interested in the comment Rob made about 'academic qualifications -Stairlift Technology. I thought perhaps there was a technical manual that is available that I could add to my library. I 'googled' that phrase but didn't gain any useful 'hits'. I'd appreciate knowing if there are any text books available to the public if there are any.

                    Thanks again for all the info you've provided in this forum on this project. I'll take some time now to digest it all.