Batteries - which charging profile

[rune]

Hi all

So, I needed a quick, dirty and most importantly cheap replacement for a pair of dead Odyssey's to get me out of a problem. So, I've bought some cheap-ish batteries. Before anyone says anything, I know they might not be that good, but it solved a problem when I needed it solved and the longer-term solution will be something different.

Anyway, these batteries are labelled both AGM and GEL. I asked the retailer which charging profile to use and they said GEL, but here's where it gets interesting.

Battery has helpfully printed on the casing:

Standby: 13.5-13.8v
Cycle use: 14.4-15v

Charger profiles are:

GEL: Absorption 28.4v, Float 27.6v
AGM: Absorption 28.6v Float 26.8v

So it seems, the GEL profile is closer to the manufacturer recommendation for Float but the AGM profile is closer to the recommended for Absorbtion.

And then there is the battery technology, Is it AGM of is it GEL? Is there any such thing as an AGM GEL combined? That doesn't seem to make any sense because you either gel the electrolyte to stop it from spilling or absorb it into a mat, but surely not both?

Thanks in advance.

Rune

[shirley_hkg]

GEL is more expensive , so cheap stuffs are AGM most likely.

[rune]

Thanks for the info.

Yes, i've been thinking maybe the AGM profile.

If only I didn't have one dead Odyssey. The other one is tested and still perfectly good. The broken one is behaving like a dead cell as it's around 2v down all the time. Thing is, this is in a chair that hasn't been used in about 5 years and I didn't realise one of the batteries was faulty, despite being kept on maintenance charge all that time. I think i've left it a bit long after purchase for a warranty claim!

[Burgerman]

Is there any such thing as an AGM GEL combined?

yes. I doubt you have those. But using a glass matt seperator instead of a plastic/nylon mesh works with gel too. Its not needed but it is just a plate seperator.

What make are these batteries, you need a proper sec sheet to find the correct charge/float voltage. Not he thing they print on the outside of a battery. Thats the acceptable limits, not the best cycle life voltages. MK will tell you 14.4, which halves the cycle life... According to their own tech data sheets.

Your dead odyssey was kept at what voltage? 13.8V which many chargers seem to think is maintainance will cook them. LONG TERM maintainance float should be around 13.35V.

Also when "tested" how was this performed?
Odyssey are good. Try discharging it, to around 95% empty slowly, with say a car headlamp bulb and doing a long slow constant current charge at 0.5A exactly for 140 hours. Then test it properly. By measuring the capacity over 20 hours down to 10.5V with a hobby charger on a discharge test. But if its dead its dead. You might be surprised though.

[rune]

Hi Burgerman

What make are these batteries, you need a proper sec sheet to find the correct charge/float voltage. Not he thing they print on the outside of a battery. Thats the acceptable limits, not the best cycle life voltages. MK will tell you 14.4, which halves the cycle life... According to their own tech data sheets.

They are TN Power TNE12-75

Your dead odyssey was kept at what voltage? 13.8V which many chargers seem to think is maintainance will cook them. LONG TERM maintainance float should be around 13.35V.

They were both kept on charge in 24v series configuration on a Xantrex TrueCharge2 charger at 26.8v for quite a time. They were then removed and charged separately.

Also when "tested" how was this performed?

By doing a slow charge and then trying to use it. I don't have the facility currently to du a proper measure of capcity, but even after a long slow charge, the battery still reads 11.6v once allowed to settle.

Odyssey are good.

Yes they are and I've never had an issue with one like this before. My experience is that they are very good, even better than Optima. I am surprised but at least I have one of them to use for other things.

Thanks for the advice

[martin007]

https://www.12voltplanet.co.uk/tn-power ... -75ah.html

Specifications


Technology/chemistry: AGM (Absorbent Glass Mat)
Application: Deep cycle/leisure/off-grid/marine/mobility
Nominal voltage: 12V
Nominal capacity: 75Ah @ 0.2C
Cycle life: 400-600 cycles at 100% DoD (Depth Of Discharge)
Self discharge: 10% loss after 3 months @20°C (from fully charged)
Terminal type: I3 (M6 female threads)
Terminal layout: +ve at RH side, front
Recommended max. charging current: 15A
Weight: 20kg
Dimensions (inc. terminals): 262 (L) x 165 (W) x 170 (H) mm
Warranty: 2 year

[Burgerman]

Cycle life: 400-600 cycles at 100% DoD (Depth Of Discharge)

If thats true, (very unlikely) then the way they did this is to use a weak electrolyte. So that heavy discharge does not damage the plates. That means high impedance. Which means your chair will be low on torque. And that the battery will be "exhausted" very early so short on range. So theres that.

Remember the battery parameters are a 3 point triangle. And all points compete, and its a compromise.
1. Ah. Good capacity means less of 2 or 3.
2.Cycle life. So good here means less of 1 or 3.
3. Internal resistance (and so hgher peukert losses). Good here means less of 1 and 2.

Its all a compromise. So at 75Ah and high cycle life, means high impedance (very bad in a traction usage job like a powerchair). May make OK standby batteries or solar battery.

[slomobile]

3. Internal resistance (and so hgher peukert losses). Good here means less of 1 and 2.

Its all a compromise. So at 75Ah and high cycle life, means high impedance (very bad in a traction usage job like a powerchair). May make OK standby batteries or solar battery.

Trying to understand the relationship between internal resistance and Peukert and noticable effects.
Using https://en.wikipedia.org/wiki/Peukert%27s_law
Check me if I get any of this wrong please.

strong electrolyte traction AGM 75Ah = rapid chemical diffusion = lower internal resistance = higher short circuit current = lower Peukert constant (1.1). When it is rapid discharged (50A) to cutoff voltage, it will last slightly longer than weak electrolyte, but is really near dead at that point. Lower Cycle life.

weak electrolyte solar AGM 75Ah = slower chemical diffusion = higher internal resistance = lower short circuit current = higher Peukert constant (1.2). When it is rapid discharged (50A) it will reach cutoff voltage sooner. If left alone for a bit, voltage will rise above cutoff and some usable capacity will remain at lower discharge rates only. Higher Cycle Life.

[Burgerman]

Correct.

With a lower impedance say Odyssey PC1500 at 68Ah 2.5mOhm you get less voltage drop under the high load a chair presents and so more of that 68Ah is accessible at the high rates we require.

So its smaller Ah takes us at least as far as a bigger 74Ah MK gel (4mOhm), and if used agressively it actually takes us further. While also allowing better torque etc, and faster charging. Remember that a 7mph chair takes us the full range of the chair (14 miles if you are very lucky, usually less) in 2 hours. But thats at a continuous low current drain. The reality is that we need much higher pulses of current to accelerate or climb a ramp etc. Or to turn in place. So the 2 hour rate is what we should look at and maybe 30 minute rate for pulses of highpower. This is where the odyssey wins. At lower rates such as the 24 hour rate the 74Ah Gel wins. But we dont use them at that rate. Also realise that this impedance increases as the battery discharges. In the case of gel "starved electrolyte" battery, like the MK drastically so. So when discharged a fair bit the internal resistance, and so voltage drop under load both increase a lot compared to say the Odyssey. They have a limited electrolyte to try to prevent serious discharge, and improve cycle life. And obviously a massive drop in performance too.

And the cheapies that have 6 to 8mOhm or worse are simply crap!

The difference in use is profound. We see greater voltage drop under load because of both resistance and the matching greater peukert values. And the chair take high intermittent loads from the battery. Say when accelerating or curb climbing. So when the control system sees the voltage drop to a low point the chair gives a red light and stops. This leaves a lot of Ah unused on a cheap high impedance battery.

[Burgerman]

When choosing chair batteries look for these two parameters first.
1. LOW INTERNAL RESISTANCE. Like the 2.5mOhm Odyssey PC1500. And a 1.1 peukert. Means it can start a truck in a siberian winter. An MK gel cant start my van in summer...

2. CYCLE LIFE. And remembering that low impedance AGMs like the Odyssey charge MUCH faster, and so get a more complete charge in the limited time available. This means you will likely see their claimed 400 cycles or more. Compared to the gel MK which charges slowly and so likely never has time to complete a charge in the overnight time we have. So likely never sees its potential 500 cycles. So reality is that both last equally well in cyclic use.

3. And the Ah rating LAST on the list. Unless lighting a bulb over 20 hours is of high importance to you. This is the one that is usually lower if the first 1. and 2. are good.

[rune]

https://www.12voltplanet.co.uk/tn-power-agm-battery-75ah.html

Specifications

Thanks Martin and BM for the comments.

I guess, if they don't work out, I'll try for a claim on that 2 year warranty. We can but try!

The longer term solution will be more odysseys or build a Lithium pack. I know which one i'd like to to but the limiting factors are time and money, both of which are in short supply for me at the moment!

Rune

[Burgerman]

The thing is lead is a bit like renting a house. Its slightly cheaper short term. And an absolutely stupid idea long term.
When you buy you do so (I did) at 1980s prices. So mortgage was very small. 25 years later, house worth 3x what I paid for it, and its mine and paid for.
When you rent, its never paid for. You are still paying, at 2022 rent prices, till you die, And its never yours.

Lithium is like buying a house. You own it and instead of replacing the lead bricks every 9 months to 18 months you forget about "rent" to keep it running. For at least a decade. Probably 2 decades if you dont care about losing some of that 70 mile range! And win all the other advantages below at the same time for free.

ALSO:
Lithium is massively cheaper Ah per Ah even today. Compared to a quality lead battery. But the main advantage of lithium WHEN DONE PROPERLY is that you can:

a. Fit in enough Ah to give 4 to 5 times the range. Yes that costs more by about double. Because you are installing the equivelent of 5 FULL SETS of lead batteries at once... So again thats literally 2.5x cheaper per "gallon" or per "mile"...

b. On top of that 4x bigger fuel tank you just fitted, you get at least a decade of serious use from them. Before throing them away because you find something even better in the future... So instead of replacing every 18 months because range is rubbish, you still have 3x the range of lead when the lithium are "destroyed"!!!

c. Fast charging
d. Lower weight.
e. MASSIVE long term £££$$$ saving!
f. Better torque and slightly higher max speed.
g. Zero range anxiety.

Fitting a 70 Ah lead brick lithium replacement gives almost non of these massive benefits.

I guess, if they don't work out, I'll try for a claim on that 2 year warranty. We can but try!

Battery warranty doesent work like that...
Trust me I used to sell batteries. They use a hing called pro rata. Look it up.

[rune]

I just thought I would check back in on this thread to let you know - the TN Power batteries ended up to be junk, exactly as was said. What has surprised me though is how quickly they started showing wear. I hoped to get maybe 9 months out of the before signs of wear - or at the very least 6, but no. They probably only did three or four months of daily use to walk the dog each day before showing noticeable issues.

Walking with my wife using the chair, after roughly 3 miles of dog walking with the occasional stop, under load the batteries just drop from full green on the battery meter to flashing red within about 5 mins. Stop for 5 mins and they come back up to high amber. It seems the voltage is sagging heavily under load and this is on nothing steeper than a slight incline. Chair is programmed and has off road tyres fitted, but is a 4mph chair.

I contacted the supplier by email about them. They didn't even bother to reply.

So there is no getting around it - it's got to be good lead or lithium. Ideally lithium. Always used used Optima or Odyssey before.

[Burgerman]

I only get 8 to 9 months from a GOOD set of lead batteries or when I had a dog at least that was true. Until I started topping up during the day to lessen discharge depth with a 40A bench power supply for an hour. Then, several years. But I wouldnt expect that from those cheap chinesium batts. You got around what I would have expected.

[rune]

I only get 8 to 9 months from a GOOD set of lead batteries or when I had a dog at least that was true. Until I started topping up during the day to lessen discharge depth with a 40A bench power supply for an hour. Then, several years. But I wouldnt expect that from those cheap chinesium batts. You got around what I would have expected.

Well I just got a pair of Optima at a good price for now. I know odyssey are better but are currently 100 more expensive each at the moment!

However, one has arrived at 12.35v the other 12.45v. BM, would you advise the 12.35 one is returned? Optima say not to store below 12.4.

[Burgerman]

I would send both back. For 2 reasons.
The optima is a good battery but the group 34 size is only 55Ah. Which isnt enough for that sized case an we are limited to that size with optima. It wont last as your average depth of discharge is too high.

And yes anything below 12.6V means its been sat on a shelf unmaintained for maybe a year. I would send both back. And expect them to be above 12.65 to 12.7V. Not 13v ish, as that shows they were low and have been "topped up" before shipping...

The odyssey is lower impedance (slightly) which is better and 68Ah. Both are pure virgin lead and electrically pretty much the same exept the odyssey is not 55 but 68Ah. Thats a huge 24% better capacity. Meaning you will have at least 24% extra remaining each day so a much lower average depth of discharge. The main thing that determines service life. Yes 100 more. Worth it.

[rune]

Thanks for the advice.

Yes, I am aware the Odyssey is better and have had them before but unfortunately I am constrained by cost and I simply can't afford any more. I have used Optima before and had good results. The long term plan is to build a lithium pack but that takes time and money and I am short of both right now. A reasonable set of batteries will buy me some time. I have started warranty claim for the Chinese ones and I hope I may get at least something back for them eventually.