Battery replacement

[prb238]

Hi Everyone -

I've been looking around for alternatives to the standard Lawnbott battery.  I am working with a battery company right now and by the way the conversations are going, it looks like they will be able to send me the components/cells to put together a battery pack with same specifications as the original battery (hopefully it will be higher amp/hrs).  They offered technical support as well if I have questions about connecting the cells together and the protection circuits.

The new battery will be LiFePO instead of Lithium Ion so it should last for more cycles.  This will mean having a separate charger so I am thinking of just taking the charging wires off the original charger and moving those wires to the new one, but leaving the signal transmitter alone.

And of course the main driver why I am doing this is cost.  It is very likely that this battery pack including a new charger will be cheaper than the replacement cost of the stock batteries.

Anyone see any issues with this approach?

[jzawacki]

Sounds like a plan to me, let use know how it goes.

[RobotLady]

It won't be cheaper in terms of liability if something goes wrong...  We have a policy not to mess around with batteries for liability reasons.  That's why we don't sell after market batteries for any products on our site.

[prb238]

Yeah, I wouldn't mess around with Li-Ion batteries which is what the Lawnbott battery chemistry is.  Most battery manufacturers won't even sell custom battery packs made of Li-Ion anymore.  I am looking at using Lithium Iron Phosphate.  Here is a quote from Wikipedia on LiFePO -

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"...... one key advantage to Lithium Iron Phosphate over other lithium-ion batteries is the superior thermal and chemical stability, which provides better safety characteristics than lithium-ion batteries with other cathode materials.[6] Due to significantly stronger bonds between the oxygen atoms in the phosphate (compared to the cobalt), oxygen is not readily released, and as a result, lithium iron phosphate cells are virtually incombustible in the event of mishandling during charge or discharge, and can handle high temperatures without decomposing.[6]
 
Lithium Iron Phosphate chemistry also offers a longer cycle life over standard lithium ion cells."
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With this safer chemistry as well as protection circuits that monitor the charge and discharge of the battery cells, I am thinking that this type of battery is safer than what is already in the Lawnbotts.

I am just letting everyone know what I am going to try - I am not encouraging anyone to go this route if they don't feel comfortable with it.

Paul

[Radjack]

What is the life expectancy of the Lawnbott Li-ion battery? how many years is common? how many cycles? how many run time hours?

I think looking at any option is a great idea, as long as the $ and headaches are worth the effort.

[jzawacki]

I haven't taken the time to dissect the lawnbott charging circuit to know if it's just as easy as "adding" an alternative charger on the wires that go to the charging plates.  What is on the other side of the charging nuts attached to the bot?  Do they go directly to the battery, or is there an on board charging circuit that will need to be bypassed/replaced?

I'm sure I'll end up learning more about it in the future when I end up replacing the crappy bump switches on the LB3200 with something better.. but till then.. I'm trying to stay ignorant.

[prb238]

@Radjack - Life expectancy is tough to answer.  My opinion is that it depends on how the battery is used, stored, and maintained.  My personal experience is that my first battery I had in my 3200 lasted for 3 years.  I knew when it was done because the robot wouldn't turn on in the charging base and the battery started to swell and overheat.  Maybe others can reply with their battery experiences?

@jzawacki - you make a good point.  I'll have to check that as well.  Usually the charging circuits (PCB's) are part of the battery packs, in fact I think it is required for Li-Ion packs.  But until we check it out, won't know for sure.

I haven't gotten any additional info from the battery company I am speaking to.  Just waiting for them to come back with a recommended cell configuration that will fit into the battery bay.

BTW - those of you out there who have 35xx robots - is there a difference between the "main" battery and the batteries that you can put in the expansion slots?  What is the maximum length of the battery compartment slots?  On the 32xx models, the base Li-Ion battery from Zuchetti is 5 7/8" x 2.5" x 3 5/8".  The max length could be up to 8", although that may have an impact on the balance of the robot - if the battery was 8" long, it would sit over the wheel motors.

[RobotLady]

3 Years is the low end for battery life on Lawnbott batteries.  We have a lot of customers who have already gone 4 years.  I hope I don't hear from them soon...   

[jzawacki]

@prb238  Now, I know it won't be as easy as you would like, as I recall reference to a change in the menu depending on the battery type back when people were having issues with mowing time.  That would lead me to believe that there is a charge controller circuit on the bot itself, which makes sense.  The on board electrics for these batteries are protection circuits, not charge controllers.

@RobotLady  From my point, that is the only thing I haven't had an issue with!  I cross my fingers hoping I didn't just jinx myself.

[RobotLady]

@jzawacki Right, I have seen the controls in the service menu.  I am not sure exactly what is happening when it is changed and can only speculate.  For sure, the processor does not communicate with the power supply, so I do not believe charging is affected by the menu change.  It is more likely that it has something to do with how the software responds to differences in the battery types.  I don't know what the differences are, so it is pretty hard to guess what would happens in the software.

PS, let's keep our fingers crossed about your battery, but bottom line, it will not last forever!  At some point it will fail, but hopefully the failure mode will be gradual rather than catastrophic because if you have a smaller yard, a gradual failure will allow you to keep using it much longer.

[jzawacki]

Hmm.. That makes sense.  I forget that the charge controller _IS_ the power supply, as it is an in circuit charger.  As you stated, the menu change is probably how it responds to the battery voltage to determine when to go back to charger, for example.

[prb238]

Yep - agree with jzawacki and Robotlady comments. My understanding is that all of the charge logic is in the charger itself. There are a few settings in the software menu that have to do with batteries - the one mentioned below that tells the robot what type of battery it is using (L-Ion or SLA) and the setting that controls when the robot returns to charging - you can set this to long (I forget the exact values) if you have a long wire and the robot will return at a higher voltage value so it has a better chance of getting home.

I have a call scheduled with the battery manufacturer this Th so I'll have more info to report. 

[jzawacki]

Well, it's Friday.. what's the word?