Charging 48Vnom battery from another 48Vnom battery

[aybabtme]

I've been looking left and right for a battery-to-battery charger that can take a ~40-60V input and charge a 16s LFP battery or 14S NMC battery on the output. I'm struggling to even find basic quality DC-DC converters from reputable sources, let alone a proper CC-CV charger. Anyone has a product in mind that can be bought off the shelf?

The use case is a "mothership" with a ~large 16S LFP battery (14kWh) that would charge a small tender's (3kWh) and eBikes (~700Wh) battery(ies) (14S NMC and 16S LFP) at a decent clip. I have in mind something about 1000W capable. Am I missing an obvious product?

 

[harrisonpatm]

Am I missing an obvious product?

Start looking at solar charge controllers. Specifically higher-end models that have reliable current limiting capabilities. I can't guarantee that every product will behave this way nicely, but for the most part, connect the battery to be charged to the battery output connection, and then connect the "mothership" battery to the solar input. In that order, please, charge controllers don't like to be connected to the solar side first, could potentially fry them. The charge controller will treat the mothership battery voltage as a solar panel with available current, and draw from it accordingly.

I have done a similar setup, as I have a 14s Li-Ion home battery powerwall setup. I used the MPT-7210 boost solar charge controller to charge my 24s LFP motorcycle battery. I have also used a simpler aliexpress 1800w buck converter to do the same.

And here's where you'll run into your problem. Your source voltage and output voltage are far too similar. In fact, you specifically mention that your source battery is 16s LFP, and one of the batteries you plan to charge with it is also 16s LFP. So technically, if your home base battery was fully charged at 56ish volts, and your LFP ebike battery was depleted at 48v, then the charge controller connection method will work, for a bit. But at some point, your charging input voltage and your charging output voltage will be the same, and no current will flow. Hypothetically call it at 52-53v, which I presume is not charged enough for your ebike battery preferences.

I don't know enough about electronics, but it's possible that someone might know enough to custom design (or at least describe) a circut that can do what you want. You need voltage to be stepped down and current regulated from the big battery source, until the voltages are the same, then you need voltage to be stepped up and current regulated. Simply because your two voltages are so close, you're not going to find a single off the shelf component that will do what you want. In fact, even with the charge controller method I described in the first paragraph, you'll see decreasing current flow to 0 as the batteries get closer in voltage.

I have a messy and inefficient suggestion. 16s LFP mothership battery -> 48vdc to 120 or 240vac inverter, 2000w rated -> plug in battery charger of your choice. This will overcome the voltage similarity problem, at the cost of 2 conversion losses and excessive expensive equipment. Sorry.

 

[aybabtme]

I have a large inverter setup so could easily plug a charger, I'm just efficiency conscious so trying to avoid the jump to AC.

I use an MPPT to charge the 12V house loads from the 48V battery so I'm familiar with this setup. The problem is that MPPT chargers need about 5V delta between the input and the output, usually. There's some MPPT that do take a wide input range (10-60V) and can charge a 48Vnom battery, but they don't look super trustworthy (Buy Buck Boost Solar Charge Controller 72V 60V 48V 36V 24V), or are very small (Genasun). Also they have tight restrictions on the max power they tolerate on the input, which makes me think they would blow themselves up by sucking too much power from the input battery.

I considered going for a 2-stage setup: (1) from 48Vnom to 72V, (2) then from 72V to 48Vnom with an MPPT charger. But these 48V-to-72V boost converters aren't so common either and their combined efficiency is probably not better than going via an AC step.

I don't know enough about electronics, but it's possible that someone might know enough to custom design (or at least describe) a circut that can do what you want. You need voltage to be stepped down and current regulated from the big battery source, until the voltages are the same, then you need voltage to be stepped up and current regulated. Simply because your two voltages are so close, you're not going to find a single off the shelf component that will do what you want. In fact, even with the charge controller method I described in the first paragraph, you'll see decreasing current flow to 0 as the batteries get closer in voltage.

This is true but that's exactly what 12/12V battery-to-battery chargers contend with and solve. There just doesn't seem to be an equivalent product for 48/48V.

 

[harrisonpatm]

their combined efficiency is probably not better than going via an AC step.

You could test it for sure, but if I had to guess, I'd say losses from 48vdc to 120vac to 48vdc, are not going to be much different than 48vdc to 72vdc to 48vdc. I don't have data on that, I'm just figuring it's close. And you said you already have inverters, so maybe take the L on efficiency in favor of using existing equipment.

This is true but that's exactly what 12/12V battery-to-battery chargers contend with and solve.

True, but lead acid can take more abuse than lithium,, and some of those even require the help of the car's alternator to boost voltage enough for charging the recepient battery

 

[harrisonpatm]

For the record, I'm very much in favor of what you're trying to do. My home's 48v goes to 2 DC converters for 12v accessories to the living room lights, router, modem, computer, tablet, and phones. Then another 48vdc line goes to 1 24vdc converter for my 3d printer. And another 12v converter for my battery testing equipment. I love keeping it DC when I can. But I'm not sure your particular case is gonna have an off-the-shelf solution.

 

[offGridDownUnder]

This is true but that's exactly what 12/12V battery-to-battery chargers contend with and solve. There just doesn't seem to be an equivalent product for 48/48V.

This is a very interesting conversation. I don't know enough yet to offer much, but I'm following. Perhaps this is a product space that is only just emerging with the popularity of personal electric vehicles and widespread personal power systems? That is, it can be done, but hasn't been needed before.

If no other solution is proposed, it at least seems to me that the inverter to AC becomes the lingua franca of safe power conversion - if only because of the historical accident of widespread use of AC and thus widespread availability of AC-powered solutions.

On a particular note, I don't know about the efficiency of inverters, but Grin claims 95% on their Satiator AC-powered charger, and it has other useful features, such as being water-safe for use outdoors. Does anyone make rugged inverters?

The solution you are asking about would be even better. I'll watch this thread hopefully.

 

[amberwolf]

It's been a while since I looked at them, but at one time Meanwell had some larger LED PSUs similar to the HLG / ELG series that could run from DC, that came in various voltage and current ranges. (there are also some tiny ones that only do less than an amp output, but the ones I'm thinking of were more like several amps output and up)

If they still have these, an LED PSU is effectively the same as a charger, except that it has no shutoff once current drops below some tiny level (which a charger does have).


Also, many "universal" (wide-voltage-range) AC-input PSUs will operate from a DC voltage within that range (higher would be better in most cases), because the SMPS input stage is a rectifier and capacitor to create a DC voltage to then operate the SMPS.

So you may be able to use an AC type LED PSU (or charger) with a DC input instead.

The biggest gotcha with this is that you are then only using half the rectifier, so you may be limited to half the input current depending on the specs of the rectifier. Or you can bypass the rectifier internally, wiring directly to the input capacitors, and can use the full input current capability .


AFAICR, DC input on the Satiator has been discussed and other than the above limitation should also work (can't find the thread ATM).

EDIT: found it, but it's 90v DC and up for the input.

The Cycle Satiator, universal charger for the enthusiasts

Some background details here: http://endless-sphere.com/forums/viewtopic.php?p=739745#p739745 Long story short, after being repeatedly frustrated as an ebike business dealing with battery chargers that for the most part sucked, we committed ourselves 3 years ago to design and develop an ebike...

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