Why is 48V battery charge voltage all different? Noob time

Batteries, Chargers, and Battery Management Systems.
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azneinstein   100 mW

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Why is 48V battery charge voltage all different? Noob time

Post by azneinstein » Jan 25 2017 4:36pm

I'm surprised nobody has started a thread with quality/cheap -non/diy- chargers as we've done the same with controllers and what not and showing the insides etc...

But I'm trying to understand why 48V chargers ranges from output voltages of 48V all the way up to some hardcore/high end ones of 58-60V.

So if I'm understanding things correctly- For example why don't we take a 48V ping battery 20AH.

Charging it with the "54.6v 2.5amp adapter" will bring it to a lower overalll capacity (80ish%) but extends many more cycles cause the lower voltage. Assuming an almost dead pack would require 8 hours of charging- 20ah/2.5amps.

Charging it with the "58v 5 amp adapter" will give me a higher overall nominal voltage and higher capacity but give me less overall cycles. It would also charge it in 4 hours.

Is my understand incorrect at all? I think my issue is that I don't understand why the total AH matters besides taking longer to charge, why does the voltage itself make a difference if I'm doing a 10 or 20ah battery? (I've seen people say this is a 48V 10ah charger or a 15ah charger)

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teslanv   100 MW

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Re: Why is 48V battery charge voltage all different? Noob t

Post by teslanv » Jan 25 2017 4:52pm

There are several factors at play here:
1. Batteries are usually specified by their nominal voltage, not their charge (Maximum) voltage.
2. Different chemistries (SLA, NiMH, LiCo, LiFePO4, etc.) will have different volts per cell, and thus different charge voltages.
3. Battery vendors recently have offered a few different voltages of batteries near the 48V range. When talking about LiCo/LiPo/LiMn, a 13-cell battery will have a nominal voltage of right about 48V, whereas a 14-cell battery will have a nominal voltage of about 52V, and both with different charge voltages.

For any given chemistry and nominal voltage, there is a specific maximum charge voltage that should be used. You CAN use a lower charge voltage, and that actually helps to keep the battery in it's "happy zone" (Not fully charged), but you can get the same effect by taking the battery off of the charger before it's done, and this has the same effect, although you don't always get the same State of charge (SOC).

Some chargers will allow you to change the charge voltage from 100% to 90% or 80%, and this is a handy feature, if it works as advertised.

The real difference maker is the charge current. - Charge current will ultimately affect the overall charge time of the battery, but is also harder on the cells, which would otherwise prefer to be charged more slowly.

It's all about finding the balance of cell lifespan vs user convenience.
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dogman dan   100 GW

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Re: Why is 48V battery charge voltage all different? Noob t

Post by dogman dan » Jan 26 2017 6:42am

What a great answer. But I'm going to add a thing that really helps me understand easier. Sometimes it helps to think of things in watt hours and watts. In watt hours and watts, voltage does not matter.

So it makes comparisons where the voltage changed easier to see what is really going on. Once you are in wh,, you can much easier see what 90% is, or 100%.

Lets take your example. 48v 20 ah ping. this is a battery that charges to 56v full. But just to make it a bit more confusing, lifepo4 is a chemistry that tolerates an overcharge. So typically it comes with a charger that is set to overcharge. so the charger is set to 58v, or with a ping, maybe even 60v. But none of that matters. everything over 56v is called surface charge, and has no capacity. if you start with a full battery and 58v, you will see 56v by the time you reach the end of your driveway.

So,, 56v 20ah is your real number, for a full pingbattery. Convert to watt hours. 56 x 20. Your pack has at most, 1120 watt hours, when it was brand new.

1120wh is your 100%. If you charge to 54.6v,, then you get 54.6 x 20 =1092 watt hours. Hmm,, not much % less is it? But now that you think in wh its easier to see the difference.

Now on to the chargers. If you convert to watts out put,, then its easy to see the rates, easy to see how that rate x one hour is now watt hours.

54.6 x 2.5 amps = 136.5 watts. in one hour, you can potentially put 136 watt hours into the battery. ( it does slow to less at the very end though)

58v x 5 amps =290 watts. So lets say you used half your battery up,, approximately 500 watt hours. At 300wh per hour, in about two hours or a little less you will be full. Two hours because it slows a bit at the end.


Charge rate does affect battery lifespan. But you have a pretty big battery. 20 ah. I would never hesitate to use the 58v charger and charge at 5 amps. But you should try to use the battery as soon as possible after it's full. If you can charge it to 90% or so, then top off in the morning, that should make your battery last longer. But charging full is good, not super harmful, if you then start discharging it right away.

It's not just charging it full, but time spent at full.

So what is 90% now anyway? Sounds like you could just divide 950 by 20, and get 90% right? Nope,, that is because the discharge curve of a cell when you graph it is not a straight line at 45 degrees. To see what voltage 90% is, you have to look at a graph of lifepo4 cell, and see what voltage a cell at 90% full is. Then since your pack has 16 cells, multiply that one cell voltage by 16.

It should be, more or less, about 3.35 volts for 90%. So around 53.6v is approximately 90% full, for new, 16s, lifepo4.

azneinstein   100 mW

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Re: Why is 48V battery charge voltage all different? Noob t

Post by azneinstein » Jan 26 2017 2:53pm

I got it now... but the way I understand it more easily is....
Take the cell voltage and multiply it by the cell count.
So in my case- 16x3.7V (I believe ping cells are 3.7v)= 59.2 Voltage that I should be aiming for.

So now my second question, if I'm using a 56.4V charger- I understand that now it would be feeding each cell 3.5V. Would the the lower voltage charger "mess up" the BMS by not supplying the original required voltage or will it balance the cells to the lower voltage?

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dogman dan   100 GW

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Re: Why is 48V battery charge voltage all different? Noob t

Post by dogman dan » Jan 27 2017 6:31am

Ping cells are 3.5v per cell, just like all lifepo4. They can charge to higher voltage than that, like all lifepo4, but that is surface charge.

Charging them to 3.7v per cell is tolerated by lifepo4 pretty well, so that's what they do. This helps your bms balance the pack better.

But,, your bms IS set to 3.7v per cell. So that is the voltage to charge to when you want to fully balance your pack.

Undercharging to 56.4 v won't hurt your bms. It will simply not start to function as a balancer till at least one cell is at 3.7v. So undercharging, you can have some inbalance develop.

Nothing wrong though, with an undercharge for many of your charges. When I used a ping two cycles every day, I ran a balance charge every night. At work, I used another charger, which happened to very slightly undercharge the battery. I would say balancing your pack weekly, if used daily, is plenty often.

I did notice that any time I drained my pings 100%,, it would unbalance them pretty severely. I'd have low capacity then till I had run several balance charges on the pack, or just left it on charge for 24 hours or so.

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