martin007 wrote:Is the apocalypse coming?
Burgerman wrote:Theres plenty of info in doing an addon pack here.
You will need to limit it to 13.8V CV and CC of 12A maximum to not damage the chairs charge connector or loom.
37Ah at 24V nominal, is 888watts. plus 10% for losses = 977 watts of energy.
Depending on what electronix/DC DC inverter you use (like a PL8 or something cheap on ebay you could use a 36V or 48V or 12V lithium nominal battery voltage. And you may lose another 10% with the inverter so now you need around 1074 watt hours needed.
To add this to an MK lead battery in 4 hours is approx 9.72 amps to the chair.
while chair (motors and electronics) consuming equivalent 1A continuously.
But that doesent happen the battery load is varied from above 100A down to a few mA when doing nothing and everything in between.
At 4 hours, the add on pack voltage should still be equal or greater than lead acid Voltage. This is defined as Pack Minimum for this application. What does that voltage profile look like?
Not sure what that sentence means at all.
Chair Maximum, Never exceed highest voltage the M3 electronics can handle, but get as close as possible. Anyone know what that is, or how to find out?
Its 35V and at around 28 volts you get regeneration spikes as you decelerate that push it very close to this limit. Which causes over voltage errors to be displayed. But you wnt to be prividing a constant current of up to whatever amps you choose, with a CV level of the batteries float voltage. And if you have MKs on board or generic AGMs thats around 13.8V per battery.
You are going to have to explain that charging part to me more carefully. Yes you can connect a LiFePO4 pack directly to a lead based chair. Many do.
slomobile wrote:.Burgerman wrote:Theres plenty of info in doing an addon pack here.
Certainly including this
https://www.wheelchairdriver.com/board/ ... e&start=60
I'm trying to define a bench testing standard to determine if a given solution is minimally successful without requiring a chair during the test. A real test would require real batteries, but batteries can be modeled for virtual tests. Bench racing. The add on pack must raise the lead acid SOC from 50 to 85% while simultaneously supplying an additional 1 amp load (carbon pile) @ 24v nominal for a full 4 hours. When the lead and lithium batteries are disconnected at the end of 4 hours, the lead SOC and amount by which lithium voltage exceeds lead is an indicator of how well it did and how much it had left to give(overcapacity). If the voltages are equal and lead SOC is still 85%, the lithium battery exactly met the minimum criteria. If lead SOC is below 85%, the lithium solution failed the test. I may have gone overboard, or chosen unrealistic values, just thought it would be useful to have a standard method for comparisons.
shirley_hkg wrote:We are group of 250 wheelchair users , mostly powerchairs.
... we have these (plug in add-ons) , to lend to them.
... The most compact one I've made is
240*110*170h , @24V50Ah. That's more than 1KWH of energy.
shirley_hkg wrote:https://www.wheelchairdriver.com/board/ ... e&start=60
slomobile wrote: What is the group? This forum? or something else?
Burgerman wrote:Explain the chair. What is its battery. What is the 2nd battery.
Then explain your plan. And why. Forget about volts and tech stuff just your working principle and why.
slomobile wrote:it was fully charged when I picked it up, so batteries should be good.
slomobile wrote:I've added a Raspberry Pi 400, 12v Project Tango Development kit tablet, small WiFi travel router, cell phone, HackRF one, to the left swingaway armrest.
If the chair batteries are getting low, I shut down the computer and reconfigure the bricks in series to prevent being stranded. Looking for your advice whether its better to connect lead and lithium together in possiby mismatched state of charge, or just run one pack or the other.
Unfortunately your method seems to require the Lithium packs connected to the chair in series all the time once the packs are mated. Is that correct?
Several of the features I was after come from your Parallel charge/Series discharge connector. Is there any way to marry these methods?
Range extension is only 1 function I was seeking. It is needed infrequently. I need the computer every day.
Ripple free 12v supply for ham electronics is another.
Ability to add range and/or computer time from 12v car charger.
Flexibility to apply stored energy to 12v or 24v systems.
The main limiting factor seems to be ensuring equal voltages as the packs are being connected to limit inrush current and inefficient operation. Correct?
Maybe there is a way to "slow" the connection process with an intermediate capacitor and charge resistors? Don't know how.
We have a Pi available onboard for cell monitoring and cell history tracking.
Perhaps the Pi could connect and disconnect individual cells, one at a time, to provide both output voltage regulation and cell balancing(by choosing which cell is modulated).
Perhaps the Pi could connect and disconnect individual cells, one at a time, to provide both output voltage regulation and cell balancing(by choosing which cell is modulated).
They are all in series with charge current going through. How can you disconnect a cell without all cells stopping charge. And the charger seeing open circuit? Same problem a BMS has. So it instead puts a resistance across the cells that reach the full point to try and discharge them a little in a futile attempt to hold the voltage down to this 3.60V point. While the charger is merrily charging away at say 10A. So that obviously fails. Then the BMS goes in to panic mode. Which it calls overvoltage disconnect. So your cell sees around 3.8 to more volts, then the BMS chops off the charger while trying to get its miserable 100mA resistive load to pull down that cell. As long as the charger hasnt decided that it shoud turn off since it cant "see" a battery, (some do) then it will keep doing this over and over untill the charger thinks its finished. Or the cells get balanced. Thats why I use a hobby charger instead because it never lets a cell go above the safe limit, and it throttles the charge rate instead.
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