Dual Battery DIY Tips

[jetetting]

I'm thinking about adding a 52v Battery to my ebike, 20amp limited on it and rated for 30amps 1000w motor with hub sensor I'm just getting into this,
Need to know if I'll be getting better performance or just blowing up my controller ?

 

[m@Robertson]

If its a 48v controller you will very likely be fine. A 48v battery is actually charged to 54.6v at 100%. A 52v battery is actually charged to 58.8v. So not much difference and the voltage will only exceed what a 48v pack produces for a short time anyway. Typically you can treat a 48v system as interchangeable with a 52v pack. There are some rare exceptions to this, but the manufacturer has to make a special effort to break the compatibility (Bafang did this with their displays, specifically to discourage aftermarket 52v packs).

You need to understand what you are going to get out of it, though. The benefit of going 52v on a 48v system is not that you are getting more power... voltage translates to top speed and you will only exceed 54.6v by a small margin. Yes you get more power but chances are you will need a speedometer to even notice the small difference. The real reason you do 52v on a 48v system is a 52v system will have a larger sweet spot of usability.

Charts can explain this a little better. Go here and open up separate windows on 52v and 48v.

Li-Ion Ebike Battery Charge Charts

Whats an 80% charge on a 48v battery? These charts give answers to questions like this on common ebike battery voltages: 36v, 48v, 52v, 60v and 72v.

talesontwowheels.com

On the charts, the second column from the left is "Usable Volts". At 100% charge, 48v is 14.5 volts versus 15.8. Usable Volts can be translated (sort of) into a gallon capacity for a car's gas tank. Figure you are going to be using power at a very similar rate for a 52v pack and a 48v pack (not quite but lets call it close enough for the purpose of understanding the issues involved). So the 52v pack gives you a bigger gas tank versus a 48v, assuming everything else is equal between two packs.

That doesn't sound like so much until you factor in stuff like the common practice of charging to 80% to preserve the pack life over time. Now a 48v pack has 11.4 usable volts versus 12.4. Its smart to preserve your battery, and a 52v system can give you a leg up on that 80% charge being enough to get around and do what you need to do on the bike without running out of gas.

You won't be setting the world on fire. The change will just work a little better and thats it.

 

[NZSimon]

Sorry but for some reason can’t open the link. That said I think I’ve seen similar charts but still have the same question - which is - “what is meant by ‘usable volts’ and/or how is this value calculated? Thx

 

[addertooth]

Don't skip the step of purchasing a "battery combiner" if you have two packs of a different voltage. Otherwise, your day will have excessive excitement.

 

[m@Robertson]

Sorry but for some reason can’t open the link. That said I think I’ve seen similar charts but still have the same question - which is - “what is meant by ‘usable volts’ and/or how is this value calculated? Thx

Just tried the link and it works, as do the chart links. Probably the most-visited page on my site so if it had gone bad I'd hear about it

Usable volts: This is pretty straightforward but I can understand how its not particularly intuitive to determine. First of all, your ebike's controller has what is known as a "low voltage cutoff" (LVC) which is the point where your controller says your battery has been drained too far to operate the controller, and so it shuts off and you either pedal unpowered, or you walk the bike.

Lets say that LVC is set to 42v, If you were using 42v as your drop-dead cutoff, and your battery is a 52v pack with the standard 58.8v max charge, then you simply subtract 58.8-42=16.8 usable volts. The plot thickens some, though. Your battery also has its own LVC built into the Battery Management System (BMS) that is built into the pack. What is that cutoff? Good question. It is probably a little more than the controller. Oh and your display may in fact also have its own cutoff setting. My KT displays have a setting that will increase or decrease the LVC in increments of 0.5v to better suit your personal preference - draining a pack down to 0% usable charge is bad for it so you might want to increase your LVC a bit.

So how did I calculate the usable voltage on the charts? As noted in the post above, for a 52v pack I certainly didn't come up with 16.8 usable volts. I came up with 15.8, and that is because the scale for calculating a 14S (52v) pack bottoms out (0.0%) at 43.0v. This is still a 6% charge on cells that have the typical individual cell range of 4.2v down to 3.0v. BUT If you drain a pack down to 0% charge you may well have killed it, which is part of why you don't want to trust an LVC on a controller (don't drain the freaking pack down so hard so you can not worry about these settings).

Clear as mud? It will be a LOT clearer if you can see the charts, where I showed all my work, all the steps as well as the corresponding cell values so you can visually do the math yourself and understand the whole process just by looking at the chart.

NOTE: I am totally ignoring the effect of voltage sag, which will cause a pack to drain down below its actual level by a volt or so, and make that LVC of 42.0 viable and not damaging to the pack. Understanding voltage sag isn't necessary to answer your fundamental question so I'm skipping that part.

 

[m@Robertson]

Don't skip the step of purchasing a "battery combiner" if you have two packs of a different voltage. Otherwise, your day will have excessive excitement.

I will argue it is foolish to use battery combiners because they are managing a critical process, so potentially dangerous it is a terrible idea to trust your house and lives to a device that, if it doesn't do a quality job, will burn down at least a room of your house. The concept of a battery 'blender' or combiner is sound but the products people are buying come from no-name Far Eastern resellers who have absolutely zero accountability to the buyer, and total ability to cut corners as they see fit... something that class of merchant is already infamous for in the battery/component marketplace.

If you want to use such a thing, buying from a vendor with a longstanding reputation, who manufactures a quality product in-house can be done, but the price you pay is something like 10 times what you pay from the fly-by-night AliExpress re-sellers.

If you want to parallel batteries, do it manually and do your homework (there's a lot of that). I've been doing it for years but I pay attention to the process right down to knowing matching cells with matching capacities (as in measuring each so you know they are all on the same side of the range of typical variance for that model of cell) inside the pack.

 

[NZSimon]

You’re a steely eyed missileman - thank you.

 

[addertooth]

I will argue it is foolish to use battery combiners because they are managing a critical process, so potentially dangerous it is a terrible idea to trust your house and lives to a device that, if it doesn't do a quality job, will burn down at least a room of your house. The concept of a battery 'blender' or combiner is sound but the products people are buying come from no-name Far Eastern resellers who have absolutely zero accountability to the buyer, and total ability to cut corners as they see fit... something that class of merchant is already infamous for in the battery/component marketplace.

If you want to use such a thing, buying from a vendor with a longstanding reputation, who manufactures a quality product in-house can be done, but the price you pay is something like 10 times what you pay from the fly-by-night AliExpress re-sellers.

If you want to parallel batteries, do it manually and do your homework (there's a lot of that). I've been doing it for years but I pay attention to the process right down to knowing matching cells with matching capacities (as in measuring each so you know they are all on the same side of the range of typical variance for that model of cell) inside the pack.

Perhaps I misunderstood, but I thought he was wanting to parallel a 48v and a 52v battery.

 

[m@Robertson]

Perhaps I misunderstood, but I thought he was wanting to parallel a 48v and a 52v battery.

He didn't say that in this thread. Maybe somewhere else.