Dual battery question

[Robotboy]

Battery question for ya. I bought a second battery, rear rack type. I use the new one primarily then unplug it, when low, and plug in the bike battery to finish out the ride. Question is, what would get you more distance, the way I'm doing it now or getting a battery combiner/blender? I haven't found a definitive answer. And can you charge them together when plugged into the blender?

 

[fabbrisd]

It will always be best to Charge each battery separately - however is is best to use a Batteyr-Combiner/Blender - allthough there are some here who HOTLY contest that. I respect Johnny Nerd Out's opinion - see his explanation below:
https://www.youtube.com/watch?v=BcEXAdPfJsY
WATCH JOHNNY NERD OUT EXPLANATION
https://www.youtube.com/watch?v=BcEXAdPfJsY

 

[pagheca]

I have watched the video, but although it is interesting (in fact, I think the original post is duplicated), I do not think that the author denies that it is better to charge each battery separately. He just says that if you prefer, you can connect both batteries in parallel with this combiner.

Actually, the only good reason I can see for preferring a combiner is that you can charge both batteries at the same time, for example at night, without having to get up to connect the second battery. But you do need a more powerful charger, don't you?

Actually, this is something I would really like, as I am planning to buy another battery for my ebike for multi-day trips: an automatic way of charging the batteries in sequence (when one is fully charged, it disconnects it and connects the next one). In this way, if I stay overnight in a hotel, I could simply connect both batteries before going to sleep and have them fully charged in the morning, without having to carry two chargers or wake up after 4-5 hours.

Is this possible?

 

[m@Robertson]

But you do need a more powerful charger, don't you?

Nope. Your increased pack size will just charge more slowly since it is now just plain bigger. Pumping up the amps on the charger is an extremely bad idea because even though you may have say a combined 14S10P pack, what it really is is a 14S4P and a 14S6P pack wired together, and it is not as robust accepting current as a 10P pack, nor can you flog it like a 10P pack and expect it to slough off the flogging like a true 10P pack could.

Is this possible?

Yes. You do it by directly connecting two batteries together - leaving them together - and charging thru one of the charging ports. The second pack will charge as the first one charges... but the current will flow thru to the second battery thru the output port connection, which - unless you have a BMS meant to cover this practice - bypasses a number of charge safety features your battery BMS provides.

This can be done safely, providing you do a slew of things right when you set the whole system up - and this goes pretty deep, involving custom packs constructed with this use case in mind. For that reason it is NOT safe for someone to do if they have to ask whether or not they can do it. The consequences are potentially catastrophic if any of it goes wrong, where 'catastrophic' includes burning down your house and killing yourself and/or your family.

So, while I have permanently parallel'd packs for many years, I don't give how-to's on doing it. BTW if I can at all help it, instead of parallel'ing two packs I have a custom larger pack built that does the job with zero risk vs. just reduced, acceptable risk.

Those battery blenders exist to mitigate the risk I am describing so unhelpfully above, so you don't have to think so hard about doing something that is inherently very risky. But here's the thing: Sure if done right, they can work (if you read the various feature sets out there, you'll see that not all of them do the same thing). But how do you know the company that made it over in China is expert enough or trustworthy enough to not cut corners? After all, Far Eastern companies who are unaccountable and unregulated as it is are already infamous for playing games. Why would you risk EVERYTHING on the assumption that you're going to find the diamond in the rough?

 

[fabbrisd]

To be frank - eBike Battery Combiners/Blender are used to safely connect 2 (or more) batteries in essence in Parallel for SAFE PARALLEL DISCHARGE ONLY .

CHARGING 2 (or more) batteries in PARALLEL is a entirely different animal - and PARALLEL CHARGING IS NOT SAFE WITH OR WITHOUT COMBINER/BLENDER

 

[m@Robertson]

To be frank - eBike Battery Combiners/Blender are used to safely connect 2 (or more) batteries in essence in Parallel for SAFE PARALLEL DISCHARGE ONLY .

CHARGING 2 (or more) batteries in PARALLEL is a entirely different animal - and PARALLEL CHARGING IS NOT SAFE WITH OR WITHOUT COMBINER/BLENDER

You are only partially correct. Some of the blenders out there specifically offer the feature of combined charging. And frankly without that you are missing an enormous benefit of parallel'd packs. Having done this as a dialy rider who needs to charge daily, I can guarantee you will occasionally forget to go out and switch charger cabling to charge the second pack. If that happens on your charger at work, you aren't going to make it home for a few hours while you stay late to fix your mistake. Same goes for getting to work in the morning. The solution is a permanently connected pair of packs that are charged as a single unit.

And furthermore, when you disconnect and then re-connect two packs together, the voltage differential between the two had better be almost nothing or, since electricity flows freely along a wire, one pack will equalize the voltage of the other in an instant flash of transferred current Which is bad. Some of these blenders are meant to address that, some are not. Its a hell of a lot smarter to connect the packs and freaking leave them connected so you don't create this opportunity to fxxk up every time you charge your bike.

For an example of a combiner that is made to handle simultaneous charging, look for the DateX2. This is probably the longest-in-the-market blender out there, and is probably the only one I would trust to actually do its job over the long term. But... if purchased thru one of their resellers, an x2 with 90a capacity and charge protection is somewhere around $500. A lot cheaper if you know how to approach the seller directly (his 'web site' is a Facebook page that leads to a Google Docs document).

Or you pay a teeny fraction of that to a no-name Far East seller on AliExpress (or buy from someone who did the same thing and marked it up to you), do without the convenience feature that is critical to day in and day out use, and hope for the best. As far as I am concerned it is a stupid trade-off. I want to just hand out the Darwin awards rather than winning one.

 

[pagheca]

Nope. Your increased pack size will just charge more slowly since it is now just plain bigger.

You are right. I said something completely stupid (I was tired, I was distracted, I was... the usual list of excuses follow ). If I can recharge in, let's say, less than 8 hours both the batteries I'm fine with it. And my battery takes about 4h to recharge from 0 with the original Bosch charger

That said, given the situation here (hilly, off-road tracks, sometimes in pretty bad conditions, etc.) my main requirement is to minimise the weight I carry. I do not care about changing batteries during the trip. So, as this unit cannot be used as a charging mixer, it will not be on my shopping list.

However, I would like to find a way to swap the batteries SAFELY and automatically during charging. Any advice?

Thanks to everyone who has replied to this topic!

 

[m@Robertson]

However, I would like to find a way to swap the batteries SAFELY and automatically during charging. Any advice?

The only thing I know of is SOME of the so-called battery blenders do their work this way: They draw from Pack A until it gets to Voltage X and then they start drawing from Pack B. I have not seen one like this for a while. I believe they were originally sold by Pasion Ebike and their USA dealers. I just checked them and its not on their site anymore.

I have also seen A-B switches that are designed to let you manually flick a switch and shift to another battery. But thats not something I would touch. Neither of these options are, really, for the same black-box concern already voiced. Even the reputable end reseller you buy from in the USA has no control over the Far Eastern producer of the product, The solution to this is to either take control of the parallel'ing process yourself, or change the game entirely and do a single bigger battery. A single bigger pack does provide you with more mileage than two separate packs that you swap. But going this route is likely not an option unless you DIY the build and have a reliable, quality battery builder/cell supplier. A manufactured ebike... you get what they sell you.

The alternative that keeps the same process you originally envisioned is to buy one that supports single charging, from a reputable (ish) seller ... who manufactures it themselves. A very short list.

 

[fabbrisd]

My understanding - for Discharge - common eBike blenders are voltage sensing - and draw from the Battery that has the highest voltage 1st/solo - and only Combine/Parallel the batteries together ONLY when the voltage is equal/matched between the cells. This s the common practice across many dual dc battery applications including applications where both batteries are different voltage/capacity.

My understanding if for correct Dual Battery Charging - the correct tech Combiner/Blender has a Intelligent Charge cicuit that sendes voltage and current - and acts as a "sequential" charger" assesses each battery separately - then charges lowest battery 1st/solo - whyen 1st is cimpletely - switches off 1st/separatel - then charges #2 - so my understanding effective dual/multiple battery charging is NOT doen in parallel