Batteries

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• 11-03-2018, 02:25 PM
landrover
Thanks. However my battery gauge rarely moves, but when it does it drops quite quickly.
• 11-03-2018, 04:09 PM
automation
Quote:

Originally Posted by Bob Sullivan
Forget the chair's charge estimate lights, they are truly rarely correct, except when it reads red. Then the "voltage" has dropped below 12 VDC.

I suspect most "fuel gauges" display some variation of the (instantaneous!) battery voltage. If the gauge truly reflected the state of the battery, then it wouldn't fall when you start to move (apply load) and then rise when you stop (remove the load). It's disappointing that an application where the "controller" can SEE the battery AT ALL TIMES and can see what it is ASKING of the battery (at all times) isn't smart enough to come up with a better appraisal of the battery's state.

Quote:

Batteries must be matched in "VOLTAGE" before you can charge them the first time.
Actually, they must have the same "state of charge". Voltage is just a crude approximation to this.

Charging batteries in parallel (without isolation diodes) is never a Good Thing as you're effectively shorting the batteries together -- the higher voltage will RAPIDLY drain into the lower.

[voltage -- aka "potential" -- can be thought of as the height of water in a bucket (a battery). The water is the charge. Water wants to flow downhill. The rate at which the water flows is the current (amperage). The diameter of the pipe through which it flows is the resistance/impedance. I.e., a thin pipe has a higher resistance than a fat one so thin causes lower rates of flow. Connecting batteries directly together is one helluva wide pipe!! :)]

Batteries in most chairs are wired in series -- so, the voltages are "stacked" (12+12=24).

If one battery has a problem (lower state of charge, shorted cell, etc.) then the charger is trying to charge the pack to ~24V but one battery is not taking up its fair share of the charge. Both batteries suffer.

(Remember, in series, everything that comes into one battery also goes through the other battery before it "exits out the bottom")

Quote:

There are some battery chargers that can tell you how many amp hours were replaced during the charge cycle. They are \$\$\$\$ but if you do the math they may well be worth the \$\$\$ to you.
The CHAIR should be doing this! Shameful for vendors to be living in the 60's in terms of battery management! The device and the user should work, together, to optimize this ("Let me know when I really SHOULD charge you and I agree to let you know what my likely usage will be so you can better inform me of charging requirements" Instead of one approach fits all users/uses.
• 11-04-2018, 03:52 PM
landrover
A bit off topic, but does heat effect battery life/performance. It seems on days when I sit next to a floor space heater, that night my battery seems to take longer to charge. Am I causing harm to my battery by doing so?
• 11-04-2018, 04:26 PM
automation
Quote:

Originally Posted by landrover
A bit off topic, but does heat effect battery life/performance. It seems on days when I sit next to a floor space heater, that night my battery seems to take longer to charge. Am I causing harm to my battery by doing so?

Heat kills batteries. Well, heat "accelerates the aging of batteries" is probably a less biased way of saying it. Every ~15-18F degree rise in temperature HALVES the life of a battery. I.e., an hour at 90F is like two hours at 75F.

Remember, batteries are "chemical reactors". Heat changes reaction rates. High school chemistry: STP -- Standard Temperature and Pressure.

Charging usually generates heat in the battery so the temperature that the battery "feels" is higher than the temperature YOU feel sitting next to it (ambient).

Sealed batteries also build up pressure while charging (yet another Bad Thing).

The issue with batteries is always one of matching (user) convenience to battery performance/longevity. If you want to be kind to the battery (i.e., get the most USABLE life from it), you charge it gently, don't deeply discharge, keep it cool (and dry -- humidity also plays a role), etc. But, this is usually counter to how the user WANTS the battery to perform. You want it to be recharged QUICKLY, tolerate your squeezing every last coulomb (bit of charge) from it, store it out in the carport when not in use, etc.

For example, a battery's nominal discharge rate is referred to as "C". Ideally, you size the battery (the size of the bucket and the "exit pipe") so that it fits your application's needs. If you want 20A from the battery, then you want a battery that is designed to deliver (at least) 20A "comfortably" (without stressing its internal design). The length of time you can operate at that rate of discharge is indicated in the battery's specifications (and this degrades, over time and "charge cycles"). Obviously, you want this to be at least as long as you are likely to need the battery to provide power between charge cycles (!).

But, the flip side of this is that the battery wants to be charged at some SMALL FRACTION of this nominal discharge rate. Typically, C/10. So, if you're expecting to be drawing 20A from the battery during use, you are also expecting to be limiting your charge current to 2A! Said another way, in an ideal battery (where what you take out can exactly be replaced by what you put back in), for every hour of use that you expect, you need 10 hours of charge time (!!).

Do the math. Use a battery for 2 hours and spend the next 20 hours recharging it. Given that you won't be 100% efficient at restoring the charge taken from the battery, you can easily see how 2 hours of use can become 22 hours of recharge.

OTOH, if you OVERSIZE the battery -- so that it can deliver 40A -- then that 20A load doesn't tax the battery as much. And, if you can charge it at 4A (i.e., 40 / 10), then the 2 hours of use can be replenished in ~10 hours, instead of ~20. (But, now you're hauling around more weight, a more expensive battery, a larger "battery compartment", etc.)

[Of course, if you can deliberately slow down your charger to deliver only 2A, then your battery will be happier -- assuming you can tolerate the time that it will take to recharge: "Sure! I'm not planning on going anywhere, tomorrow!" A smart appliance would watch to see how much "energy" it is using and know how aggressively it would have to recharge -- IF IT KNEW THE USER'S FUTURE NEEDS! Imagine if your chair "remembered" that Wednesday is your "busy day" -- lots of use -- but Thursday is a "slow day": "Let's take our sweet time recharging, Wednesday night, as we've got very little need tomorrow!"]

You just have to come to an accommodation between your "desires" and your "finances" -- trade one for the other as you see fit.
• 11-04-2018, 10:33 PM
RollPositive
Landrover...1st week try and charge 12 hours you need 15 charges for the batteries to be broken in and always charge every day even if not using.

After the 1st week you can charge for 8 hour a night and be good.

Did you get the larger 24s or still using the 32's?

Once a month they and get on 15 hour charge.

Also we use a small electric throw blanket that plugs in keeps me much warmer and heats faster then sitting by a heater.
• 11-05-2018, 04:29 PM
landrover

A electric blanket would not work for me. I'd be stuck plugged in somewhere.
• 11-05-2018, 04:53 PM
gjnl
Quote:

Originally Posted by landrover

A electric blanket would not work for me. I'd be stuck plugged in somewhere.

Not to mention, you can get a nasty burn from electric blankets, throws, and mattress covers when they are resting on an area where you can't feel the heat.
• 11-05-2018, 05:38 PM
RollPositive
Quote:

Originally Posted by gjnl
I had this discussion with a Permobil representative at the Abilities Expo in San Mateo last week. She said when deciding when to charge your wheelchair batteries take several things into consideration. Generally speaking under "normal" use conditions, it is a good idea to charge the batteries every night. However, if you didn't go much distance in the chair, say, just round your house, there may not be a need to charge the batteries if the next day is going to be a similar light use day. But, if you know that you will have a heavy use day, even though the indicator lights don't show the need for a charge, it is a good idea to charge them up anyway.

The picture below is the controller I use on my Permobil M300. The red, yellow and green lights across the top indicate battery charge. If you have had a light use day today, plan to have another light use day tomorrow, and the last two green lights have changed color and indicate that you don't have a full charge, it will probably be okay not to charge the chair after use today. But, if those lights indicate you don't have a full charge and you will have a heavier use day tomorrow, it is a good idea to charge those batteries today after use. Lastly, it is a good idea to charge the batteries after any heavy use day.

The Permobil rep you talked to has no idea what she is talking about!!

Per MK:

-----Original Message-----
From: Belinda Perez [mailto:bperez@mkbattery.com]
Sent: Monday, November 05, 2018 2:13 PM
To: RollPositive
Subject: Batteries and charging

Hello RollPositive:

That is correct you should charge your batteries daily after use. I recommend charging at least 8 hours if possible and maybe 10 once a week.

Let me know if you need further assistance.

Belinda Perez
Customer Service Representative
MK Battery
714 922-2025 (Direct line)
800 372-9253 (Customer Service)
714 937-0818 ( Fax )

"placing an order just became easier!"
www.mkbattery.com
• 11-05-2018, 05:43 PM
RollPositive
Quote:

Originally Posted by gjnl
Not to mention, you can get a nasty burn from electric blankets, throws, and mattress covers when they are resting on an area where you can't feel the heat.

Why would you use an electric blanket if you have issues with not being able to feel sensations that would harm you?

Give the possible user some acknowledgement of having common sense.

Think about if the same could happen sitting in front of a heater and being to close...
• 11-05-2018, 05:49 PM
RollPositive
Quote:

Originally Posted by landrover