stupid question about SLV's and Lipo4

[naz]

hi there, i currently have a razor drifter with SLA's (i am also looking to convert other gocart/kids motorbike)
anyway, the SLA's are with a 24v system and not sure about the motor or the amps..

can i swap the sla's directly over to lipo's? obviously i would need a different sort of charger - but other than that it should all be the same shouldn't it - providing i change it over to a 24v and the same if not higher amps.

the other thing - some times it is just better to swap out everything except the motor if it doesn't cost too much, thoughts??

[Psi]

You will need to address the problem of over-discharging the lipos

[naz]

over discharging?? i thought the BCM (mini circuit board that came with the lifepo4's)would cover that?
i didn't think the controller took care of that?

[peter.mitchell]

over discharging?? i thought the BCM (mini circuit board that came with the lifepo4's)would cover that?
i didn't think the controller took care of that?

To quote myself;

Protection modules are usually designed as a failsafe, not as a normal means for charge/discharge management (eg, you don't turn your TV off at the breaker box when you're done, you put it in standby, or you turn it off at the wall maybe).

In commercial devices, there are usually multiple layers of protection, for discharging; software to put device into standby/shutdown, then if the device continues to draw power beyond a charge threshold there is a charge controller to disconnect the load, then over time the battery will self discharge and also discharge slightly from the charge controllers circuitry and other attached leakage, this is where the protection module cuts off everything from the cell.

For charging; there is usually some software to set the charge rate in the charge controller, then the charge controller charges the battery according to its specs (at the current set by software, with failsafe hardware limit, to a specified voltage according to spec), if the battery continues charging beyond that voltage, there is a protection module to cut the cell off from the charge controller.

[naz]

ah, ok... learn something new everyday

so what sort of device would i need to put in for discharging or charging?

the reason this surprised me so much was because i got a kit to convert a pushbike and as far as i could tell there was no added device to protect discharge except the bcm - and while i was riding if the battery got too low it would just cut out (literally).
i would turn the key off and on again (to reset it) but once i put the motor under load, it would just cut out again..... once recharged it would be all good.

again what device do i need?

i would thank both of you, but there is no thanks button so thanks

[naz]

did an internet search, probably using the wrong search words.

could you give me a link to the article or a link to the device?
thanks

[Psi]

It's likely that many china products rely on the built-in battery protection when they shouldn't.
Torches that take lithium cells is probably a good example.

You really do not want to discharge a lithium ion/polymer below 3.0V.
The built in protection doesn't usually kick in until 2.8V, or even 2.5V, so it's more to protect the cell from a chemical reaction that can make the cell dangerous to recharge.

3.3V is a good point for software to consider the battery completely flat as you get minimal extra capacity discharging lower than that. Some device might use a higher voltage, like 3.6V, to trade off some capacity for extra lifespan, they may also charge to 4.1V instead of 4.2V for the same reason.

A product using lithium batteries might have the following states.

Voltage between 4.2V and 3.3V - Normal operation
* device would report battery full at 4.2V and empty at 3.3V

Voltage below 3.3V - Battery flat
* The device may flash a low battery icon if you try to use it, so it's still running but in a low power state

Voltage below 3.0V - Battery critical
* The power circuitry in the product disconnects the load from the battery to prevent damage, the product appears dead.

Voltage falls to between 2.5V and 2.8V - Failsafe
* Internal battery protection kicks in and disconnects the battery from the product.
This prevents unsafe chemical reactions from occurring.
The battery health is being seriously damaged every day it remains in this state. A few weeks like this and the battery may become useless.

[naz]

wow Psi,, that is such a good explanation  just what i needed to explain. thanks

i would really appreciate a device name or words i could use for a search - so i can look into these items with a view to buy

[nukie]

Hello Naz, Psi

Psi probably didn't catch that you are planning to use LiFePO4. So the numbers he provided is not suitable for your use.

The LiFePO4 chemistry can take abuse lots. I have swapped my NiCD cordless drill and portable vacuum to LiFePO4 run them down to almost flat, CV balanced charge at 3.45V per cell. 3.6V is ths recommended max voltage but 3.45v to limit the charge to around 90% or so. I have done this over many times for a few years now, I found no degradation in capacity but slight increase in internal resistance, around 3%. I top it off regularly so I never have to concern about overdischarging the cells. For your once a while use I suggest that you store the batteries flat, then charge them up before use.

If the nominal voltage of the LiFePO4 pack is close to your SLA then why not. You can choose 7S pack or 8S for a bit more grunt at the expense of motor life. If you choose 7S, you will feel it when the batteries go flat at 2.8V per cell, LiFePO4 has a very sharp knee drop when discharged. There are some hobby chargers capable of charging 8S packs.

You will have a lot more fun minus the SLA batteries weight. And you will learn more about LiFePO4.

Hit up endless-sphere.com there are more people with experience with what you are doing there. They can provide help and point you to the right direction regarding the charging and motor drive electronics.

[peter.mitchell]

The LiFePO4 chemistry can take abuse lots.

Whilst they can take more abuse than the regular LCO or LMO chemistry cells, I wouldn't say they can take "lots", nor would I say it is solely to do with the chemistry.

Packaging makes a large difference; cylindrical cells are far more robust than their pouch cell brethren, their metal case provides a far stronger enclosure to the cells, reducing physical damage and removing the need to "clamp" or "squeeze" the packs. Often when referring to cells taking more abuse, people compare across different chemistries and across different packaging somewhat invalidating their claim.

Manufacturer makes a large difference; dodgy, no-name manufacturers may have poorer quality control, less stringent testing, a higher level of acceptable defects. People often compare across different chemistries and manufacturers somewhat invalidating their claims.

Eg; An A123 LFP cylindrical cell vs a Turnigy LMO pouch cell instead of A123 LFP cylindrical vs pouch or a Samsung LCO Cylindrical vs a Ultrafire LCO Cylindrical

You will have a lot more fun minus the SLA batteries weight. And you will learn more about LiFePO4.

I dunno, i think in this application the weight difference will be pretty negligible, his "gokart" uses 2x 7ah 12v in series, they are about 2.1kg. an equivalent replacement with LFP would be 3p8s, if you use A123 26650s, which weigh 74 grams each, that would be 1.776kg vs 4.2kg, or a difference of 2.424kg, not much at all.

And then there is the price... even with price estimates in favor of the A123 cells, it still looks bad, over estimate the price of a pair of 12V7Ah as being $50, and the A123 cells being $110 (about $9 ea).

In this application, I'd say the upgrade value is somewhat dubious.
I repeat though, In this application.

[nukie]

I'm pretty sure the LiFePO4 beats the SLA deep discharge in terms of charge/discharge cycles. But not suitable in this case since the OP is has also taken cost into consideration.

I built a new pack out of Turnigy nanotech LiFePO4 seems to hold up fine. Its used to run a 18V 5" grinder which most people know they are hard on batteries. I wouldn't suggest they are great as I don't have enough data but the cylindrical A123s are true to spec in my older battery packs. I would deploy these cells for high demand applications.

Obviously i wrote in great favour of Lithium technology with my limited experience with portable tools.

[peter.mitchell]

I'm pretty sure the LiFePO4 beats the SLA deep discharge in terms of charge/discharge cycles. But not suitable in this case since the OP is has also taken cost into consideration.

Indeed, personally, I'd just stick another 2x SLAs on it in parallel with the existing ones, an avoid deep discharging it; more use time, better burst current delivery no extra fancy stuff needed. Extra 4.2kg isn't that much.