36/48V Lead Acid ---> 48V Lithium

[wpns]

I have a number of 48V (and a couple of 36V) lead-acid golf carts, and I've just converted one of the 48V carts to (2) https://alliedlithium.com/products/allied-lithium-48v-x-30ah-batteries batteries. And have discovered (in addition to the inaccuracy of the supplied amp-hour-counting meter, sigh) that there's no warning for running out of battery, the cart just stops dead. Turning it off and letting the battery recover for 10 minutes gives you another 10 feet of mobility. [One advantage to Lead-Acid, it just keeps getting slower, so you can usually limp home, though at the expense of deep-discharging your battery pack, which is really bad for it.]

I did borrow some jumper cables and connected one of the 48V carts for 15 minutes and managed to get home, but I'd really prefer a more general solution. (current limiting? What current limiting?). I'd like to source a 48-volt (36V-60V?) input power converter that'd deliver up to 15 Amps (5-30A?) to a "48V" LifePO4 battery for future use. Doesn't even have to do any kind of voltage-based charge stages, just try for an appropriate voltage and limit current till I have enough charge to get home.

Obviously it's not going to be as inexpensive as a set of jumper cables, and extra points for using a golf cart to jump-start EG4 wall-mount batteries, so a more general solution isn't bad.

This kind of thing must exist, no?

 

[HarryN]

There are a couple of possible ways to improve the situation.

Step one is a more accurate "fuel gauge" for the power left in the battery pack. The ones that are based on voltage will not really work with Li batteries, instead you will need a monitor that measures the actual current flow in / out of the battery pack. The measuring device to do this current flow is called a "shunt" and it is installed in the negative wire just before the battery.

The measurement is turned into useful information with the monitor / display. Several companies make them for 48 volt use including victron and bogart engineering. Either one will work just fine.

Victron tends to be focused on using your cell phone for the display.

Bogart offers it with the display in a box about the size of 2 snickers candy bars, and a smaller separate unit for the wireless.

The choice is very much a personal aesthetics decision - I have used both in 48 volt AGM and LiFe systems and they both work just fine and are accurate enough for your use. Officially the bogart is more accurate but it does not matter that much for a golf cart.

- Bogart Engineering Battery Monitors

Bogart Engineering is the manufacturer of Trimetric & Pentametric brand of battery monitors & solar chargers. We're a team of engineers & scientists with experience in solar & battery technologies. If you researching batteries or solar technologies for use in your RV and have questions or need...

www.bogartengineering.com

Battery monitors - Victron Energy

The Victron Energy battery monitors monitor the charge status of your battery and ensure a uniform charge status.

www.victronenergy.com

 

[HarryN]

For charging from 120 vac, there are a number of 48 volt chargers on the market. I kind of like the IOTA engineering units but there are multiple threads on the forum - just do a search and type in 48 volt charger.

If there is room, of course the easiest way is to simply double the size of the battery pack.

Here are two on line retailers that sell 48 volt battery chargers.

NAZ Solar Electric | Off Grid & Grid-Tied Solar Power Systems

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Battery Chargers, DC Converters, Inverters, Solar Chargers | 12v Through 144v DC Equipment | ChargingChargers.com

Battery chargers, DC converters, inverters, solar chargers to maintain flooded, gel cell and sealed lead acid batteries. Commercial quality, microprocessor controlled equipment at affordable prices.

www.chargingchargers.com

 

[HarryN]

This company makes a small solar charger for 48 volt golf carts. It is enough to slightly extend the range of the existing battery pack.

And yes I have experience with them and they work just fine. It was in my minivan - not in a golf cart but same concept.

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sunforgellc.com

Those are sold by a few places including alte

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www.altestore.com

 

[LakeHouse]

[One advantage to Lead-Acid, it just keeps getting slower, so you can usually limp home, though at the expense of deep-discharging your battery pack, which is really bad for it.]

The only real difference here is that this is actually waaaay worse for LFP than it is for Lead-Acid. So much so that LFP batteries have a battery management system (BMS) that monitors the individual cells and physically disconnects the battery when the voltage reaches a point where damage could occur.
That's why the cart just stops; the BMS has disconnected the battery. And yes, if you leave it for a few minutes the cell voltage will increase very slightly and the BMS might reconnect, but there's still no real energy there so it'll just disconnect again if you try to drive the golf cart.
The upside is that if the BMS does its job correctly (as it sounds like it is), it doesn't degrade the battery, so you can operate right up to the point of exhausting the charge and not cause damage.
But, as @HarryN notes, you need to monitor State of Charge with a tool built specifically for that job, usually a shunt. Voltage-based monitors that work for Lead-Acid just don't work for LFP.

inaccuracy of the supplied amp-hour-counting meter,

Could you provide us some details on what this device is? If the golf cart already has a shunt, it might just be the case that it needs to be properly calibrated.

 

[LakeHouse]

Victron tends to be focused on using your cell phone for the display.

This is true for the SmartShunt, but the BMV-7XX devices have displays that can be remotely mounted.

 

[wpns]

Yeah, I have the shunt-style battery meter, but I've got a 48->12V converter that runs the lights and radio, and when it's turned _OFF_ it still draws a couple hundred mA from the battery. However, this is _not_ enough for the shunt to see any power leaving the battery, so after a while the battery capacity is lower than the meter thinks it is.

Yes, I have a relay to completely disable the 12V converter, which should(?) solve that particular problem, I just need to connect it to the key switch, etc.

 

[Rocketman]

All the negative wires (including the 48-12 converters) need to flow through the shunt. The Victron BMV712 will measure and track all currents. (Even very small ones).

If you are running out before the shunt says zero your batteries are not delivering as much power as your monitor thinks it will - reduce the Ah’s on the monitor. (Or you may need to top balance the cells better). Or if you have 12v batteries in series- one may be shutting off earlier and need more top balance or more charging.

Good Luck

 

[wpns]

Victron BMV712

Looks interesting, if a bit pricey. The video says "Make sure you fully charge your battery at least twice a month which ensures the BMV can synchronize and show the correct state of charge." I've already got a system that's out of sync in a week or so, is this going to be that much better? Or is the above just a CYA?
Also, the BMV700 is a bit less, is there much point to the Bluetooth and 'secondary measurement'?

Thanks!

 

[Rocketman]

For programming alone - the bmv712 is worth the price difference over the 700.

It is much easier to program plus you get to see all the other data it collects. (Which if you dive into it can tell you a lot).

 

[LakeHouse]

Make sure you fully charge your battery at least twice a month which ensures the BMV can synchronize and show the correct state of charge." I've already got a system that's out of sync in a week or so, is this going to be that much better? Or is the above just a CYA?

The shunt measures current flow and infers SoC from that. Over time, small errors can add up. The only conditions where the shunt actually knows the SoC is when the battery is full or empty, so periodic full charge resets it and eliminates those accumulated errors.
For reference, my Victron shunt drifts by about a few percent per week, so when it resets it’s usually only correcting an error of a few percentage points.

 

[LakeHouse]

For programming alone - the bmv712 is worth the price difference over the 700.

It is much easier to program plus you get to see all the other data it collects. (Which if you dive into it can tell you a lot).

Agreed. Programming with the display is a pain.