Is there a way to do controlled battery discharge?

[raylo32]

So, it happens once in awhile... I plan to go on a ride and charge my battery 52V EM3EV Super Shark fully, and then something comes up that prevents me from riding. Rather than leave it like that for a week or 2 I'd like to be able to discharge it to an intermediate state. Is there an easy way to do this without riding the bike or hooking it up to a trainer?

 

[E-HP]

The dollar store still carries old fashioned incandescent light bulbs, so that and a desk lamp may be the simplest solution, assuming that you'll be monitoring and disconnecting once you reach the desired storage voltage. You can also get fancy and add a cheap LVC module to cut it off automatically.

 

[rafalg]

Is it better to discharge and then charge again than just leaving it charged?

 

[E-HP]

Is it better to discharge and then charge again than just leaving it charged?

The battery's capacity will degrade the longer it stays at full charge (or discharge). If you have to fully charge, it's best to go riding right after taking it off the charger, but since the OP had something come up, he wants to drain it down to a storage level charge (which is sort of a half charge).

 

[raylo32]

I have an RC LiPo discharger but I don't believe it can handle 14s... so, for now it is the fluid trainer and a piece of duct tape to engage walk assist for an hour or so. Then I'll check voltage.

 

[Comrade]

Is there an easy way to do this without riding the bike or hooking it up to a trainer?

Yes, you can just buy a large power resistor for a few dollars on Ebay/Aliexpress/Amazon.

 

[harrisonpatm]

Yes, you can just buy a large power resistor for a few dollars on Ebay/Aliexpress/Amazon.

This would be the easiest way. You'd just need to do math on it. You said you have a 52v battery. What's the full capacity?

 

[eMark]

Rather than leave it like that (FULLY charged) for a week or 2 I'd like to be able to discharge it to an intermediate state. Is there an easy way to do this without riding the bike or hooking it up to a trainer?

Wouldn't you be able to ride the bike for a few minutes that same day or week for partial discharge (95%). One reason some only charge their 14s to 90-95% (53-56v).is for prolonged cycle life. When needed you can always give it a FULL charge later when you have the time. Shouldn't take longer than an hour (depending on charge rate).

An 18650/21700 ebike battery can be stored at 90% (or even 95%) for a few days ... assuming the BMS, bus bar connections, cells, wires are AOK. That's why some prefer using a [smart] active balancng BMS for monitoring the parallel groups to prolong the cycle life of their DIY fabrication ALAP..

 

[E-HP]

Regardless of what alternative method you use, you still need to get rid of the heat safely. A socket base and 200W bulb will dissipate a couple of amps for $6.

Amazon.com

Westinghouse 200 W A23 A-Shape Incandescent Light Bulb Medium Base (E26) White 1 pk Mfr# 0397300 - Ace Hardware

Incandescent A23 and PS30 shape light bulbs are perfect for general lighting purposes that require high light output. These light bulbs come in clear or frost finishes. To help you find the right bulb for the right application, see Westinghouse’s offering of A23 and PS30 shape light bulbs...

www.acehardware.com

 

[amberwolf]

Or a toaster oven, or other resistive heater, etc.

 

[Hillhater]

Search for Dr bass and his cheap $10 hair dryer discharge set up.

 

[raylo32]

Sounds good. Only thing with any of these resistive load approaches is that I need to make up an XT connector and cable to fit it up. One of these days...

 

[Hillhater]

Poor's man 19kW load resistor !

Time for sharing a great idea for when it come to test battery power and energy at high rate !! :twisted: ! u-qo6sjzS-Q Doc

endless-sphere.com

 

[amberwolf]

Use the battery connector of your choice wired to the wall-type side of an extension cord, or even build up an actual outlet into a double-space wallbox with faceplate, and a switch in the second space. If you think you will need to parallel several loads, build a triple or quad wallbox up with as many paralleled outlets as may be necessary.

Then you can unplug the "loads" when done and use them for their normal duties.

 

[raylo32]

I just ordered a 300w 10 ohm power resistor that will provide a nice safe ~5 amp discharge, or about 250 watts. I'll fasten that to a board or maybe make a 3D printed mount. Then add a pigtail with XT plug.

 

[batteryGOLD]

So, it happens once in awhile... I plan to go on a ride and charge my battery 52V EM3EV Super Shark fully, and then something comes up that prevents me from riding. Rather than leave it like that for a week or 2 I'd like to be able to discharge it to an intermediate state. Is there an easy way to do this without riding the bike or hooking it up to a trainer?

Do not store any battery fully charged (90%-100% soc) for long time periods (more than a few days), it will reduce battery capacity/life and not safe.
The simplest way to discharge battery is buying a alluminium 1000W 4ohm resistor, cost around $50 inc shipping from china

Doing some math. 52V battery on a 4ohm load will generate 52/4 = 13Amperes continuous discharge current.
This means U putting out 52Vx13A= 700Watts dissipated in heat while dischargin battery. (this values change beacuse 52V battery operates from 58,8V down to 37,8V, but 13A is average discharge current )

Example,
if you have a 26Ah battery, using a 4ohm resistor, it will theorical full discharge battery in 2hours. (13Ax2hours equals 26Ah )
But if U just want to discharge a battery from 100% SOC to 50%SOC, just discharge durin aprox one hour, or measure volts to know SOC% level

a 52V battery is a 14S , soo a 100% charged battery equals 4,2Vx14= 58.8V , 80% soc means 4,0Vx14=56V, and nominal voltage 52V is at 50%soc (3,7V per cell, reccomended level for storing)
a 0% soc level, means all cells at 2,7V ( this is the lower battery voltage cut limit 37,8V)

 

[batteryGOLD]

The dollar store still carries old fashioned incandescent light bulbs, so that and a desk lamp may be the simplest solution, assuming that you'll be monitoring and disconnecting once you reach the desired storage voltage.

your ideia usin incandescent bulbs is great and a lot of light.. but U cant simple just chose a random lamp from thousands of lamps and connect to any battery..
first U need to do some math and ohms law..

example 220V 100W bulb
U=R*I , P=U*I than R=(U^2)/P means R= 48400/100 equals R = 484ohms

if you connect tha bulb to a 52V battery, discharge current will be I=U/R --> I=52V/484ohms means 0,1A discharge rate..
soo imagine dischargin a 26Ah battery at a 0,1A rate, means 260hours discharge time..

12V 100W hallogen car lamps could discharge tha 52V battery
R= 1,44ohms each one
but you cant connect a 12V bulb to a 52V battery, it will explode!
U need to calculate the number of S bulbs for that volts level.
means 52V/12V equals 5 bulbs in series needed to get a 60V bulb array 500W

this 5S bulbs have a final R of 1,44ohm x 5 equals 7,2ohms rated at 500W light ouput

soo if U connect five 12V 100W incandescent bulbs in series and connect it to a 52V battery, it will draw a current of 52V/7,2ohms equals 7,22A battery discharge current and a lot of light output!!!

It will discharge a 26Ah 52V battery in just 4hours and putting a lot of light out!
carefull with some car 12V hallogen lamps, because emit a lot of UV visual radiation!!

 

[raylo32]

I have already done the math. The 10 ohm resistor is perfect for my smaller batteries. One hour discharge will take them down to about ~60% SOC.

Do not store any battery fully charged (90%-100% soc) for long time periods (more than a few days), it will reduce battery capacity/life and not safe.
The simplest way to discharge battery is buying a alluminium 1000W 4ohm resistor, cost around $50 inc shipping from china

Doing some math. 52V battery on a 4ohm load will generate 52/4 = 13Amperes continuous discharge current.
This means U putting out 52Vx13A= 700Watts dissipated in heat while dischargin battery. (this values change beacuse 52V battery operates from 58,8V down to 37,8V, but 13A is average discharge current )

Example,
if you have a 26Ah battery, using a 4ohm resistor, it will theorical full discharge battery in 2hours. (13Ax2hours equals 26Ah )
But if U just want to discharge a battery from 100% SOC to 50%SOC, just discharge durin aprox one hour, or measure volts to know SOC% level

a 52V battery is a 14S , soo a 100% charged battery equals 4,2Vx14= 58.8V , 80% soc means 4,0Vx14=56V, and nominal voltage 52V is at 50%soc (3,7V per cell, reccomended level for storing)
a 0% soc level, means all cells at 2,7V ( this is the lower battery voltage cut limit 37,8V)

 

[harrisonpatm]

I just ordered a 300w 10 ohm power resistor that will provide a nice safe ~5 amp discharge, or about 250 watts. I'll fasten that to a board or maybe make a 3D printed mount. Then add a pigtail with XT plug.

Don't make a 3d printed mount; you did the math right, so you should be expecting 250-300w off that resistor. I bet that'd be enough to melt any mount that you'd 3d print.

 

[batteryGOLD]

Regardless of what alternative method you use, you still need to get rid of the heat safely. A socket base and 200W bulb will dissipate a couple of amps for $6.

seems low cost bulbs..

• Volts: 120 volt
• Watts: 200 watt

R=72ohms
soo if you connect to a 52V battery discharge current will be 52/72= 0,72A ..

but if you parallel x10 of that bulbs, R will be 7,2ohms , this leads to a 52/7,2=7,2A battery discharge
but price will be 3.99 x10 equals $39 for this light discharge setup

 

[raylo32]

I know. A good bit of heat. I'd use PETG and mount the resistor on insulators.

Don't make a 3d printed mount; you did the math right, so you should be expecting 250-300w off that resistor. I bet that'd be enough to melt any mount that you'd 3d print.

 

[mpetry912]

Careful here people. Don't burn your house down discharging your batteries.

 

[batteryGOLD]

Careful here people. Don't burn your house down discharging your batteries.

maybe, if they use hallogen car lamps..
hallogen bulbs temperature wikipedia:
540 °C (1,004 °F)

safer to discharge usin resistor load, maybe temperature goes up to 150 celcius

 

[amberwolf]

Do not store any battery fully charged (90%-100% soc) for long time periods (more than a few days), it will reduce battery capacity/life and not safe.

It is perfectly safe.

Lifespan reduction, sure...but there's nothing unsafe about it.

 

[amberwolf]

Don't make a 3d printed mount; you did the math right, so you should be expecting 250-300w off that resistor. I bet that'd be enough to melt any mount that you'd 3d print.

I guarantee it would, since many common plastics soften at 150F and can melt not that far above this (some take twice that to fully liquefy, but most will do so much lower just takes longer). 3D printer filaments almost always are designed to soften and/or melt at fairly low temperatures in these ranges (there are some that require better/hotter printheads but they're not the commonly-used ones).

A typical soldering iron, that easily and quickly reaches 700-800F, only takes 15W. (a 15W light bulb's glass surface temperature won't get that high because it's too large an area for the heat generated inside at the filament and the distance of vacuum between them, but a resistor has direct conduction from the heat source to the much smaller surface area, so even a 15w resistor could soften or even melt some plastics if enclosed by them).