Battery replacement

[dakota77]

Hello
I'm building a robotic platform based on an Puma 40 wheelchair and Permamobil control.
The 2 batteries are old and must be replaced.
Could this battery be suitable for a wheelchair ?
https://onlinebattery.be/fr/categorie/optima-blue-top-marine

Thanks

[Burgerman]

I used to sell optima.
What you need are the light grey ones. The dark grey cases are starter only and not deep cycle capable.
So light case marine blue top, any yellow top, and non of the red top ones.

But what you really need is the odyssey pc 1500 battery as it is greater capacity, equally deep cycle capable the same or better low impedance and the same physical size.

[dakota77]

Thanks for your reply.
odyssey pc 1500 is much more expensive...
https://www.onlinebattery.be/fr/fiche/odyssey-pc1500
What I understand is classic car battery is not convenient. It must be deep cycle capable.

About the 2 batteries I'm using :
Fully charged : 12.8v
Discharged : 11.7v
I have remarked that one battery discharges lower than the other.
I charge using independently using a 12v charger.
It takes only half an hour for a complete charge which seems very low. I suppose that indicates that the battery is defective.

Is there a method to see if the battery is still good ?

[Burgerman]

Where to start.

Yes deep cycle. But a lead battery isnt really good enough anyway even if you fit the best. And as you noted the best isnt cheap. You CAN fit the smaller 55Ah odyssey but that will just have a higher average discharge level daily. So will die faster.

You can fit some super cheap eBay AGM batteries. But they are junk.

Or you can fit lithium.

One advantage you have is that you may have space for a bigger Ah (say grp27 or grp31 100Ah battery which of course will be more lightly loaded and have a lower average level of discharge so will live longer. Much longer.

Chargers...
If the current batteries are Gel, then anything over 14.1V @ 20C will be killing them at at least double the speed... Your 500 cycles will be 250 give or take. Thats what most mobility chargers do. And most generic 12V chargers.
Any charger that tells you a lead battery is charged in less than 6 hours (already full) or around 16 hours (if discharged) needs to go straight in the bin. Because its not posible to charge any lead battery fully which is important for longevity in cyclic use any quicker.

How to test?
Fully charge each PROPERLY and then measure Ah out. Theres a number of ways o do that. I use a hobby charger that discharges at the 20h rate to 10.5 or 10.8V as specified and then read off the Ah.

You can do it with a heaadlamp bulb, and a voltmeter and a clock. Etc.

You also need an AC impedance meter. Really. But chances are they are ruined. Unless new.

[Burgerman]

About the 2 batteries I'm using :
Fully charged : 12.8v
Discharged : 11.7v
I have remarked that one battery discharges lower than the other.
I charge using independently using a 12v charger.
It takes only half an hour for a complete charge which seems very low. I suppose that indicates that the battery is defective.

These batteries should read over 13V after charge plus an extra 12 hours wait. IF properly charged. If they go from 11.7 to 12.8 in an hour then they are toast. Unless you are using a 100A charger. Like a car alternator.

[dakota77]

Thank you for this very comprehensive and informative response.
I use a NOCO GENIUS 5 Output 12V 5000mA charger. So I think the problem is not with the charger. I'm interested to learn that it takes an average of 12 hours for a full charge and should read more than 13V !

I also bought 2 other new batteries: Optima Blue top BT DC 4.2 55 Ah. I noticed that with these new batteries, the autonomy of the robot was very short: +/- 30 minutes. One of the 2 batteries was discharging very fast. I sent it back to the supplier who confirmed that it had discharged too much and that it was out of service.

So I'm wondering if I should buy just one that won't be the same age as the other or I should buy 2 new batteries .
It seems to me that the batteries which are connected in serial, must be identical (type, age and so on...).

The second one seems to discharge normally but I will make the test you suggested : a converter 12V to 220V and a lamp. Then I will measure the time it takes to recharge it.
How low can I let it go ? In other words at which voltage can I consider that the battery is discharged ?

Thanks again for your time

[Burgerman]

What voltage s discharged?
There is no particular point.
Some manufacturers like to use 10.2V when discharged at the 20 hour rate. Some 10.5. Some 10.8. Many have spec sheets that will specify a capacity for all three of these points, and also at different discharge speeds like 10h or 5 hours etc. The faster you discharge the less you get. Its not a linear fuel tank.

Your charger like most cheap generic chargers has about 7 stages. We always only want 2. Or 3 in the event that it ends CV stage too soon. Which yours does. How do I know? You tell me it ends charge after 30 mins! That MUST be at least 8 hours at CV total 12H even on a battery that is almost full. And 16+ hours on a battery that is discharged heavily.

Remember that you get maybe 100 cycles (3 months use) if you drive the chair till it grinds to a halt. From the BEST battery.
You get maybe 350 cycles (a year) if that same battery is discharged regularly to 80%
But thousands of cycles if its only discharged to say 50%. In all cases when charged PROPERLY! Which your charger doesent do.

A 55Ah battery like yours (deep cycle blue marine optima?) will give 55Ah out, discharged to 10.2V IF CHARGED CORRECTLY over a 20 hour time period. So to do this accurately you need a load that will end the charge at around 20 hours. Give or take an hour. My charger does this automatically. But I can do this manually as described too. But I need to monitor it and mess about with calculating loads.

A discharge test.


For e.g you need a 2.75A load. For 20 hours. Thats 55Ah out. Every hour wil remove 2.75Ah. A battery that has dropped to 80% of the rated capacity is considered end of life. But you notice a loss of performance (torque) long before this - well I do. You only need a 12V bulb and ohms law to arange that current. No inverter needed.


Myself, I just program my charger to DISCHARGE at 2750mA and tell it to stop at 10.2V.
Then I get a curve, as well as a mAh out reading.

See below.

[dakota77]

Very interesting. I'm going to test my batteries.
Especially the Optima.
I didn't know the voltage could go that low.

Eventually could you give the reference of a good charger ?

Thank you very much for your time.

[Burgerman]

Good charger. Every battery varies slightly so a one size fits all doesent work. Anything that claims to have a million marketing "stages" or is "automatic" or universal, claims to charge gel/AGM etc is all nonsense and just marketing.

I use a bench power supply set to the correct Amps and CVvoltage and stop charge manually, or a ZXD programmed to charge how I tell it, or the hobby PL8v2 that is completely user programmable and that charges any chemistry, cell balances lithium etc as well. The answer isnt simple. The real answer is that you need to use a little knowledge.

A good 2nd easy guess would be a victron charger that may be 12 or 24V and various currents. With a FULL custom profile setup using their PC software.

[dakota77]

Ok
PL8v2 seems a very good device.
thank you

[Burgerman]

Of course any old charger will charge any lead battery. It all depends on how much you care about doing it properly so that in cyclic use it lasts as long as possible. That is important to a wheelchair user especially as a set of these is ££££ and the problems caused by lack of range is highly problematic if it stops you from doing your daily routine.

And even more so if you are a heavy user. Even charged properly I used to get 10 to 12 months at best from lead. But I murdered them by doing heavy discharges every day. Many users (the vast majority) according to stats, seem to spend more time drinking coffee and watching TV. The average user never does more than a mile or two daily. But if you are active, as a good few are on here then lead just doesent cut the mustard. Thats why I started using lithium 15 years ago. You can rapid charge it, and sized correctly will give decades of use, and anything from 3 to 5 times the range depending on the size you fit. Currently its possible to fit 230Ah in place of the standard sized GRP 24 chair battery.

230Ah is huge.
When you consider that a 75Ah lead battery only actually gives us around 45Ah at best due to peukert after whch the chair stops. Which lithium doesent suffer from. You get to use all of its Ah. So 230Ah div by 45Ah = 5.12 x the original range. So your original 20 mile is now 100+ miles. With 10 to 20 year lifespan instead of a year. So 5x he stored energy, 10x the service life, for about double the cost... And its around 40% lighter too.

[Dan]

What will you be using the robotic platform for? Because without all the seating and the weight of a person I would think that even cheap batteries (just check the size, that they will fit) will last for hours of use. Free second-hand car batteries will also get you up and running
Cheapish AGM. There is probably better out there.
https://www.ebay.co.uk/itm/134430738832 and a shitty 24V charger https://www.ebay.co.uk/itm/116094288522 for two 12v batteries in series.

[Burgerman]

Thas true. If its a project for fun, that will only be used internmittently and you dont care about cycle life any old battery and charger will work.

[dakota77]

Thanks for your comments
You are right, the usage here is completely different.
The robot has many uses : the first is cleaning surfaces with a high pressure cleaner, spraying with an electric sprayer, pulling a thermal mower, pulling a trailer, transporting loads, and so on.
Currently I am encountering the following problem: I have 2 55 Ah Invacare batteries. The robot's autonomy is only 30 minutes at very slow speed after practically no charge apart from its chassis. After 30 minutes the battery indicator on the remote control flashes red. If I measure the battery voltage: battery 1 = 12.5v - battery 2 = 11.3v
Obviously battery 2 is defective. The problem is that the batteries must be identical because serial connected.

As you suggested I will try to find 2 similar car batteries to see if the autonomy is better.
Thanks for the very useful the links

[Burgerman]

You will get around 20 to 50 cycles from sli starter batteries.

And invacare dont make batteries. They put some stickers on so you hink they do.