Putting RC Lipo in series with a BMSed battery - do i need a diode?

[neptronix]

Yeah.. i'll be charging a battery with a BMS and a RC battery separately, but i plan to discharge them in series.
I will be using regen.

Was wondering if a diode is needed here, and if so, what function is it serving?

 

[harrisonpatm]

I will be using regen.

Therefore, you will indeed be charging them in series, as well as discharging. Therefore, any diode you put in the circuit is going to interrupt current flow for either charge or discharge. I think that's right anyway.

So what you're going to want to check is whether the BMS in the (presumably?) larger battery can handle the full series voltage, once you put the RC lipo in series. Do you have a datasheet for it?

 

[E-HP]

I’m not using a diode, but have meters measuring each voltage. I bulk charge through the discharge port of the combined setup, so it should work similarly for regen charging.

 

[neptronix]

I’m not using a diode, but have meters measuring each voltage. I bulk charge through the discharge port of the combined setup, so it should work similarly for regen charging.

Do you have two BMSed packs?

 

[E-HP]

Nope. 14s with BMS, 6s lipos without.

 

[neptronix]

Therefore, you will indeed be charging them in series, as well as discharging. Therefore, any diode you put in the circuit is going to interrupt current flow for either charge or discharge. I think that's right anyway.

Funny i didn't think of this but you're right!

So what you're going to want to check is whether the BMS in the (presumably?) larger battery can handle the full series voltage, once you put the RC lipo in series. Do you have a datasheet for it?

Is this well known to be a requirement for series'd BMSed packs? all the advice i've read in the past says you just need a diode..
However the function of the diode, or the 'why' has always eluded me.

 

[neptronix]

Nope. 14s with BMS, 6s lipos without.

..yeah i thought it worked like this.
Do you happen to have the 14S BMS pack at the bottom of the series chain starting from negative BTW?

Wondering if that even matters..

 

[beemac]

From what I've read on here - i think the BMS needs to be able to handle the voltage of both batteries should some sort of failure occur.

 

[harrisonpatm]

From what I've read on here - i think the BMS needs to be able to handle the voltage of both batteries should some sort of failure occur.

This is the main issue. I've never heard the advice regarding using a diode. But it doesn't matter in which order they're seriesed: the BMS is going to see the full circuit voltage, and if its components are not rated for said voltage, they'll be damaged

 

[docw009]

From what I've read on here - i think the BMS needs to be able to handle the voltage of both batteries should some sort of failure occur.

Here you go guys, the search engine came thru, The whole story is there from where the guy asks about a series connection to where he reports explosions at night from his bike, The cautions he received about the BMS were pretty low key, but it's clear to me he fried one of them and lost BMS protection,

My ebike caught on fire.

I have a 52v battery and I’d like to get to more volts. Im thinking about getting a 36v battery. So, 52+36=88v. My controller is rated for 72v. I’m curious if the battery BMS will conflict with wiring different voltage batteries in series and do the batteries need to be the same amp hours...

endless-sphere.com

 

[E-HP]

I never charge unattended, and monitor all pack voltages during the charge. Even if it isn’t a non standard configuration. Downside doesn’t make it worth the risk. I’m charging at a fairly high rate though. I’m actually ordering an 8s monitor so I can check cell level voltage on the lipos since i bulk charge.

“Charging the battery for a few days fixed the issue. While riding around with both batteries in series I noticed the battery that caught fire was getting noticeable hot, but the other battery had a normal temperature. After leaving the bike charging for a few days that’s when the fire started. I’m unsure why the fire started.”

 

[DogDipstick]

In the early days of industrial mechanical control battery reserve systems (1900-1910, 1913ish,) the battery banks has what was called " booster cells"... cells that were mechanically switched in at will to keep voltage where you want it........ here a boost, there a boost... add a cell... subtract a cell.. was the job off the battery tender ( was human at this time).

6s bank of (X) aH capacity lipo on top of the 14s battery of ( ? ) aH capacity BMS? Is it the same, more, or less capacity than the rest of the ( 14s) pack?

Gee IDk.

 

[DogDipstick]

I never charge unattended, and monitor all

I run my chargers on a wall timer.. one of those little dial things.. .tickticktick.. ticker timer. Turn dial to 1hr, charger will absolutly disconnect from wall mechanically after one hour ( sw in timer).

I also use a 300A USB shunt monitor battery wifi thingie ... measure amps thorughput, watts, aH, V... and sends it to a little USB screen plugged into computer... so I can see the energy transfer sitting in the room mext door the charger.... At the computer... and watch the gauge for when the charger stops in the other room.

 

[E-HP]

I run my chargers on a wall timer.. one of those little dial things.. .tickticktick.. ticker timer. Turn dial to 1hr, charger will absolutly disconnect from wall mechanically after one hour ( sw in timer).

I also use a 300A USB shunt monitor battery wifi thingie ... measure amps thorughput, watts, aH, V... and sends it to a little USB screen plugged into computer... so I can see the energy transfer sitting in the room mext door the charger.... At the computer... and watch the gauge for when the charger stops in the other room.

If I need to step inside for a bathroom break, I leave a network camera on and the door open, so I can see or smell things (other than the bathroom) for 5 or 10 minutes. I also always set my phone timer for 20 minutes (even though I'm present and I usually charge for an hour or a bit more total) just to remind me to not get distracted. My battery setup probably scares folks. 28Ah 14S lithium ion in parallel with 5Ah 14S lipo, in series with 24Ah lipo, all bulk charged at 20S at 7A or 8A. I manually dial down the charger to 3A when everything gets a little over 82V or so and stop at 83V. I can go several dozen charges before I may need to slightly top up either the 14S or 6S so they both reach 4.15V/cell at the same time, but the cell to cell voltages haven't gone out of balance, so they seems to be pretty well matched. I never really go below 3.75V per cell, so no issue on the discharge side.