Need battery help

[bill_delong]

This is probably the first evidence I've seen which supports an issue I experienced and most folks do not believe me when I share my experience.

Back when LiPo's first came out the C ratings were very low making IR far more volatile and unfortunately, IR checking wasn't a thing back then so I had no clue what the health of my batteries were other than age. Average lifespan was about 3 months for a battery before they started to swell and crack the casing. That was my indicator to replace a pack which is very sketchy.

Anyway, I replaced one of my 2S packs that had severely swollen with a brand new pack that was unmatched with an aged pack that wasn't swollen. Different brand battery and different C rating though same capacity. I think the old pack was a 25C and new pack was 20C, I figured it was "close enough". Everything seemed fine for the 5 min quals, but the 10 min main would cause the brand new pack to swell up and crack the case on the very first race day!

That would be the last time I would ever run 2x2S in series.

Fast forward 10+ years to today with 120C as industry standard, I don't think running packs in series is quite as dangerous as it used to be, however I have taken temp readings on a friends saddle pack after a long main and one battery was 20°F hotter than the other pack which was enough proof for me to understand that packs in series causes more strain overall and will require your batteries to be replaced sooner.

I would not recommend running packs in series without checking the IR at the end of every charge cycle, this will give you peace of mind.

 

[RustyUs]

What about using 2P built packs with 1P built packs? You know, those xP2P packs that use two cells in parallel to make higher capacity packs.
2S2P = actually four cells inside
3S2P = actually six cells inside
4S2P = actually eight cells inside

 

[bill_delong]

Unfortunately the SMC article doesn't go into enough specifics about "WHY" packs in series causes issues, but I firmly believe it's a variable based on the length of the wiring between the packs and connector quality. There are many modern saddle pack designs with a very short jumper cable, and someone who properly maintains their connectors to minimize IR by spreading the pins to increase lateral grip strength of the connection will yield the best results.

When you have a single pack with internal solder tabs with virtually no IR whatsoever, then you won't introduce the risk of premature wear between the cells.

That said, virtually every single battery I've owned tends to have the S1 cell on true negative lose IR before any other cell which holds true to xSxP theory still experiencing the same phenomena though running packs with external series connectors will compound the IR and make the external pack on the true negative side degrade faster as it gets worked harder.

I have chosen to keep things simple and avoid the risk of increasing IR with added connectors and wiring with packs in series.

YES, packs in series will work, however the pack on true negative side is stressed harder... packs in series won't last nearly as long as a single pack based on my personal observations and sharing information with other racers in my area who have been running saddles for many years.

 

[Greywolf74]

Unfortunately the SMC article doesn't go into enough specifics about "WHY" packs in series causes issues, but I firmly believe it's a variable based on the length of the wiring between the packs and connector quality. There are many modern saddle pack designs with a very short jumper cable, and someone who properly maintains their connectors to minimize IR by spreading the pins to increase lateral grip strength of the connection will yield the best results.

When you have a single pack with internal solder tabs with virtually no IR whatsoever, then you won't introduce the risk of premature wear between the cells.

That said, virtually every single battery I've owned tends to have the S1 cell on true negative lose IR before any other cell which holds true to xSxP theory still experiencing the same phenomena though running packs with external series connectors will compound the IR and make the external pack on the true negative side degrade faster as it gets worked harder.

I have chosen to keep things simple and avoid the risk of increasing IR with added connectors and wiring with packs in series.

YES, packs in series will work, however the pack on true negative side is stressed harder... packs in series won't last nearly as long as a single pack based on my personal observations and sharing information with other racers in my area who have been running saddles for many years.

I totally agree with this.

When two LiPo batteries are connected in series, the one on the negative end of the circuit typically bears more stress and wear than the one on the positive end. This situation arises due to the nature of electrical current flow and the intrinsic properties of LiPo batteries. Here's a technical explanation of why this occurs:

Electrical Flow and Stress Distribution:​

• Current Flow: In a series circuit, identical current passes through both batteries. However, the battery connected to the negative side, referred to as Battery A, is the initial recipient of the incoming current from the load. As a result, it must supply all the electrons that flow through the circuit. Battery B, positioned on the positive side, 'receives' these electrons after they've moved through Battery A and the load.
• Initial Discharge Stress: Battery A, being the first to discharge, experiences a marginally higher initial load compared to Battery B. This increased load, over repeated use and time, can lead to accelerated degradation, particularly in the cells closest to the negative terminal.

Internal Resistance (IR) and Heat Generation:​

• Internal Resistance Variation: Even brand-new, identical LiPo batteries can exhibit slight variations in internal resistance (IR) due to manufacturing tolerances. A battery with a slightly higher internal resistance will generate more heat for the same amount of current.
• Heat and Stress: The battery on the negative side, already under more stress from being first to discharge, will suffer more if it also has a slightly higher IR. The additional heat generated can accelerate wear and degradation. Heat speeds up chemical reactions inside the battery, leading to quicker capacity loss and reduced lifespan.

Voltage Drop and Imbalance:​

• Voltage Drop Across IR: A higher IR in Battery A will result in a more significant voltage drop when current flows. This means that, under load, Battery A's voltage will be lower compared to Battery B, potentially leading to imbalanced discharging where Battery A reaches its low voltage cutoff sooner.
• Cumulative Effect: Over time, the small differences in discharge rates and the additional heat generated can lead to Battery A aging faster than Battery B. This manifests as reduced capacity, increased internal resistance, and a greater likelihood of cell failure.

Considerations for Running Mismatched Cell Counts:​

Running batteries with mismatched cell counts (different voltages) in series, even with identical IR, can significantly exacerbate the issues mentioned above:

• Voltage Imbalance: When batteries of different voltages are used together, the lower voltage battery will be forced to 'work harder' to keep up with the higher voltage battery, leading to increased stress and potential over-discharge.
• Monitoring and Management: If mismatched cell counts are used, it's crucial to monitor each battery's voltage closely. Employ individual cell voltage monitoring and Low Voltage Alarms (LVAs) for each battery to prevent over-discharge and damage.
• Consistent Usage and Charging: Ensure that the batteries are used and charged consistently. Even with the same voltage, variations in usage and charging can lead to imbalances over time.

@Greywolf74 we should start another page in the lipo spread sheet and experiment and document what I have done so far with running a 4s and 2s in series.

That's all you, buddy. Pick the ball up and run with it!

 

[nino]

You can run a lipo alarm on each pack. Not an elegant solution. Or reliable if it disconnects mid bash.