Burgerman wrote:Theres plenty of info in doing an addon pack here.
Quite true. So much that it gets confusing sorting through the threads. Lots of times I think I am reading through solid info, only to find its an old thread with out of date info. I guess the older threads, being older, float to the top of search results, causing them to rise even further. Sticky helps, but the sticky threads get very long and polluted, and are also old info. Some sites compile the most relevant old info in a new thread, lock the thread from new comments, then sticky that. Its ok, I'll figure it out, was just going direct to question given the short timeline.
You will need to limit it to 13.8V CV and CC of 12A maximum to not damage the chairs charge connector or loom.
Then I'll use Anderson pigtails to the batteries and maybe add a presence detect wire to an XLR cable adapter to increase the current limiting in case I want to make it universal for other chairs.
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.
Thanks, that gets me closer. Probably use solar mppt charger with programmable cut out.
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.
The additional 1A draw is just an estimated value picked out of a hat to represent all loads on the chair other than battery charging. Like quiescent current, seating actuators, brief driving, but with the current spread evenly over 4 hours for easier analysis.
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.
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.
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.
That is extremely helpful knowing where the spikes rise to. 35v is a pretty common discrete component voltage tolerance which I was expecting, but confirmation is better. I may slightly exceed 13.8v per battery as it wont be above that level very long, and sluggish programming and driving should mitigate the regen spikes. Thats just me saying I'm willing to risk my batteries. Not saying it wont kill them prematurely. I've heard that gel AGM are susceptible to localized overheating when bubbles in the gel electrolyte don't collapse when overcharging But that isn't a factor in their life expectancy when kept to 13.8 because the gel resists bubble formation in the first place.
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.
I'm second guessing now. I thought I needed to be extra careful that the add on pack voltage was not too high, so I was thinking of ways to top balance at a lower than normal level. Maybe not necessary. What is the maximum pack voltage of the LiFePO4 pack directly connected to lead batteries? Do they insert any current limiting? I was looking at the possibility of avoiding current limiting by judicious choice of lead charge vs lithium discharge curves at less than maximum lithium SOC.