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Old 17th December 2016, 06:46 PM   #1
albertNL is offline albertNL  Australia
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Batteries
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As a wheelchair user I looked up how lead-acid batteries work earlier this year. Perhaps it is useful if I share some things I discovered because a lot also applies to battery based PSU's.

1. A battery has more than one "capacity" (in Ah). Often values are given for C20, C10 and/or C5. These give you the capacity if you were to drain the battery at a constant current in 20, 10 or 5 hours (usually down to 80% discharged or a specified voltage).
The total number of Ah's that can be used decreases with the rate at which the current is drawn. So, only when you know how much (constant) current you will be drawing will you be able to estimate (by looking at battery datasheets) how long it will last.
Look up Peukert on google for more info (or confusion).

From experiences in wheelchair use, it has been found that you should get the largest battery that will fit in your wheelchair. This is also due to the fact that the current demands of a wheelchair vary immensely from one moment to the next. I average about 1.3Ah per km of travel (normal outdoor travel at about 12km/h using a 24V wheelchair) but currents vary from negative (braking recharges the batteries!) to peaks of 130A accelerating uphill from standstill.

2. Lifetime in max. number of charge/discharge cycles is usually specified for 80% discharges. When a battery is recharged after only 20% discharge the number of cycles increases by more than the expected factor 4. If you recharge after only 5 or 10% the life increases spectacularly.

3. A cheap standard car battery is fine for a pre-amp that draws a small constant current (e.g. FirstWatt B1) but you would probably rather use a Gel or AGM sealed battery. No maintenance or possibility of spillage.

- if you draw a small constant current an MK Gel is the best choice. They last long (number of cycles) and have a low self-discharge rate. Disadvantage is low max. charging current (overnight charging for wheelchair users).
- If you draw high or varying currents an Odyssey Extreme series AGM battery is the best choice (extremely low internal resistance). They do not offer as many cycles but can deliver more sustained amps and also have a low discharge rate. They can also be charged with a much higher current than MK Gel (making quick half hour 40A or 60A charge top-ups possible).
If not used very often (i.e. number of cycles is limited) then MK Gel will last about 7 years. Odyssey AGM will last 10.
Note that MK AGM batteries are not worth considering.

4. Whatever battery you buy, if you do not charge it correctly you will not get the Ah or cycles specified. A standard charger is intended for a generic (non-existent) battery. Either build your own charging circuit or make sure that your charger is fully programmable.

Charging at a voltage that is too high can cause the escape valves to open when pressure inside the battery increases (decreasing lifespan) and if unchecked may even cause thermal runaway (fire).

If you use MK Gel batteries and charge using a standard charger you will reduce the number of cycles dramatically. Max. charging voltage at 20 degrees Celsius is 2.35V per cell (so 14.1V for a 12V MK Gel battery). Charging at 14.8V (standard chargers sometimes go even higher) will result in a 60% decrease in number of charging cycles. And yes, you will have to use temp compensation to reduce voltage at higher temps (and increase at lower temps).
Also, watch out you do not recharge at a high current. MK Gel cannot handle it. But, if you use Odyssey AGM batteries and charge at a low current you will reduce number of available cycles.

==> So, program your charger to get the most out of your battery.

5. Wheelchair users are switching over to LiFePO4. Advantages are lower weight, increased number of charge/discharge cycles (2000+ vs. 400) and as the batteries discharge there is hardly any voltage drop and current can be maintained. Disadvantage is that charging is much more difficult (balanced charging) to do right. This is due to the fact that each cell (usually bought separately) only delivers 3.2V (nominal), overcharging can cause fires/destroy cells and discharging to a very low voltage damages the cells as well. There are special charges that can take care of the charging problems, e.g. Cellpro PowerLab 8
DIY wheelchair builders and modders usually do NOT use a so-called BMS (battery management system) because these can cause a lot of headaches and have been known to cause fires. I have heard that Boeing's 787 battery fire problem turned out to be BMS related. A fire is not good for a wheelchair user who cannot get up when his chair is on fire!

If you decide to use a BMS (specially if it is an unknown cheap bms) make sure you understand how it works and decide if you really need it. An intelligent charger handles the charging issues much better than any BMS can. If you need BMS discharge protection perhaps DIY options (current limiter, voltage determined shutoff) are a better choice.

An easy option where you avoid a lot of the problems would be to use higher voltage prismatic cells. Example of a 40Ah 12V prismatic cell Winston LiFeYPO4 12V 40Ah Cell [WB-LP12V40AH] - $249.00 : EV Assemble, LiFePO4, Electric Bike Conversion Kit, EV Charger, BMS, EV Components, EV Parts, All for EV!

Fire hazard means Li-ion and Li-polymer batteries are a no-no for wheelchair users.

I hope this info is of use to someone.
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Old 18th December 2016, 05:20 AM   #2
PRR is offline PRR  United States
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Thanks.

This may be especially useful to "buskers", street musicians. They are always coming up with battery amps, often poorly engineered. (Some can't be helped.)

We should also decide if the job is weight critical. Self propelled vehicles (cars or wheelchairs), the battery mass works against the main goal (gaining speed or going uphill). Buskers have to carry the weight to the gig and back. OTOH a 100 pound preamp battery only ever moves twice (to install, and to recycle).

All your points about size, capacity, rate of discharge, and life accord with all I have found over the years. Your comments on new chemistries and chargers are valuable.

> A cheap standard car battery is fine for a..

In my opinion, the potential for acid-fumes rules this out. (There's actually laws about lead/acid inside structures, though nobody will arrest you for a car battery.) The old days, batts threw fumes whenever charged. Improved car systems and low-maintenance batts have reduced the problem in cars. High-rate/voltage chargers are still the standard for out-of-car lead/acid charging.

I bring our sport-car and lawn tractor batts inside for the winter. But I charge outside, let rest a day, and then put them in cellar near a door. (No, concrete does not discharge a modern plastic case battery.)

One of these is "MX5 Miata" style. The original was truly sealed. The replacements are vented. I have never seen one vent. But I sure would set the batt in tupperware or similar if it was in the nice part of the house.

There are a couple "alarm batteries" which are sealed lead/acid of good capacity, and often stocked in the home-store.
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Old 18th December 2016, 07:24 PM   #3
albertNL is offline albertNL  Australia
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Batteries
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Originally Posted by PRR View Post
We should also decide if the job is weight critical. Self propelled vehicles (cars or wheelchairs), the battery mass works against the main goal (gaining speed or going uphill).
True, but heavy batteries also results in a low center of gravity. Some wheelchair users have had to add weight below their LiFePO4 after moving from lead/acid.

Quote:
In my opinion, the potential for acid-fumes rules this out.
You are right.

The problem for most wheelchair users is that the bedroom is where you charge your wheelchair while you sleep ...
Venting of Gel or Agm should not occur unless the charge voltage is too high, if you use charge correctly (which many do not).
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Old 19th December 2016, 03:36 AM   #4
PRR is offline PRR  United States
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> low center of gravity

Indeed, a key point for a not-poor performance vehicle with payload (person!) center of gravity roughly as high as the wheelbase.
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Old 19th December 2016, 04:50 AM   #5
trobbins is offline trobbins  Australia
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It's a pity that the low power end of the Li-Ion market appears to be awash with cheap BMS, and that diyers are trying to cut dollar corners by compiling cells and aftermarket BMS.

I recommend trying to identify and use battery/BMS combinations from mature reputable manufacturers as a safeguard against poor (or even disastrous) performance.

A battery manufacturer's own BMS, coupled with a Li-Ion module of appropriate voltage and kW/kWh ratings, and that integrates the necessary voltage and temperature sensing, along with system level current sensing, and charge/discharge and over-current protective facilities, offers peace of mind and almost bullet proof performance (well at least nail proof).
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