I apologize bc I'm sure this question has been answered in here before, but I cannot find it. This is really a General question about the difference in Lithium Batteries. I read someone talking about doing a conversion from FLA system, and that's what I'm considering and doing researching on. The comment was that convert to Li-ion batteries not LiFePo4 batteries, but no explanation given.
So, What is best? What is the advantage of Li-ion? I already understand that there is a fire hazard with Li-ion, but it seems that LiFePo4 is the preferred battery. Is it that the Li-ion just more available to DIY projects? I'm starting down the DIY path and I'm at a fork on that path. Of, course Yogi Berra would say "Take it."
Any info or point me to old conversations would be appreciated.
Thank You, Joe
Definitely one single 48v battery as it will be managed by a single BMS that will keep all cells balanced. 4 x 12v batteries is asking for trouble. You have 4 BMS’s all, doing their own thing and your batteries will (not may) become unbalanced. This will reduce your run time more and more over time. When this occurs, The fix is to top balance by fully recharging each battery individually with a 12v charger. But they will then drift out of balance again over time.
More important than capacity is maximum continuous discharge amps, maximum peak discharge amps and the time that the maximum peak amps can be drawn for. Unlike FLA batteries, lithium batteries have a BMS that will limit current to protect the cells. It will disconnect the battery and shut the cart down if the parameters are exceeded. These maximum figures will be dependent upon your carts configuration, but I personally wouldn’t go for anything under 150A continuous and 250A peak for 20 seconds for an unmodified cart.
Choose the correct battery for your cart and you’ll never regret the added initial cost. Performance is so much better, not to mention virtually no maintenance and long term costs will be less as lithium’s should last you at least 10 years.
Cheers
Pat.
You should never connect lithium batteries in series if you want a reliable solution. I know 4 x 12v batteries in series equals 48v, that’s not the issue. There are 4 BMS’s all doing their own thing. The issue is no two batteries are ever equal, there are tolerances.
With Lead Acid batteries in series, when one battery becomes fully charged, the charger keeps charging. The fully charged battery is overcharged while the other batteries catch up. With Lead Acid, it’s no problem. Lithium batteries on the other hand absolutely cannot be overcharged so when the first battery in the string gets to full charge, its BMS disconnects to prevent overcharging. Since the batteries are in series, the first BMS that disconnects will stop charging all together, therefore, the other 3 batteries will never reach full charge.
The energy stored in a series connected lithium battery is only ever as good as the least charged battery. In the above example, assume you have 100Ah batteries, but when the first battery cuts off at full capacity, the least charged battery only has 80Ah in it. Well, your 100Ah is now only 80Ah as its BMS will disconnect once this battery is discharged to 0% prevent damage. As you go through more and more charge/discharge cycles, this capacity difference will increase, reducing the pack capacity further.
This can be fixed from time to time as SOK suggests by fully charging each battery individually using a 12v lithium charger, but they will drift out again, sometimes in as little as months. Lithium is touted as being maintenance free, having to top balance manually every few months certainly isn’t maintenance free! A single 48v battery with a single BMS will keep the internal cells in balance.
Parallel connected batteries on the other hand are less troublesome but still not advised if possible. The issue here is that if one pack’s BMS disconnects while the other keeps going you will never know. Also, if the two pack are connected together when they are not at a similar SOC, very large currents will flow between them, possibly tripping off the BMS and preventing a parallel connection.
Parallel connected batteries are ok if you understand how they interact, but series connected batteries are definitely a no no.
Cheers
Pat.
I’m not sure about the concern for multiple batteries versus one in regards to their becoming unbalanced. SOK battery says it isn’t a concern for batteries in parallel. They do say,however, if the batteries are in series they need to be equally charged.
Hey fella's I'm in total agreement. I was responding to Neoflyer saying he's not sure there should be concern for multiple batteries becoming unbalanced. He's quoting SOK battery, but that quote is backing up what we're saying. To get 48 volts you don't run 4 - 12's in parallel you run them in series, and with 4 different BMS's they will become unbalanced.
Single point of failure stops you cold.
I don’t subscribe to that in this case. The whole point of the BMS IS to stop you cold under fault conditions. Lithium cells are dangerous when abused, but perfectly safe when used within their design specifications. It is far better to have one battery sized correctly for the task at hand.
Firstly, You may think you have redundancy with two batteries but in this case, you’ll end up with two smaller batteries that cannot produce the current to drive the cart on their own. If one BMS fails, The remaining BMS will be continuously tripping on high current or you’ll be drawing more current from the smaller cells than they’re designed to supply.
Secondly, having one BMS fail, and having the second battery trying to push current into it (they’re connected in parallel don’t forget) could be catastrophic, especially if the BMS fails to a short circuit. The cells in the battery with the failed BMS are now totally unprotected while receiving very high charge current from the second battery.
Cheers
Pat.
I forgot to add, if reliability is your concern, then one properly sized battery will be orders of magnitude more reliable then two or more insufficiently sized batteries in parallel.
There are two correct ways of increasing the capacity of a Lithium Battery Pack. First is to use larger cells. Second is to use smaller cells in parallel to create a larger capacity cell, then connecting them in series to get the voltage you require, managed by a single BMS protecting all cells. That is exactly how the cells in electric cars are configured, they certainly don't have multiple seperate batteries in parallel, each with their own BMS. The latest Tesla Model 3 has cells in a 31p96s (31 cells in parallel then 96 of those 31 cells in series) configuration but only one BMS.
Cheers
Pat.
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