Battery aging, internal resistance, and end amps

vtmaps
vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
I've been thinking again, so I am confused. I have read here and elsewhere that as a battery ages its internal resistance increases. I have also read that as a battery ages its absorb end amps increases. This doesn't make sense to me. At a given absorb voltage I would expect (ohms law) that as internal resistance increases (with age of battery) the current (end amps) through that battery should decrease.

Am I wrong about end amps increasing in an aged battery? Am I wrong about internal resistance increasing in an aged battery? Am I wrong about ohms law? If I'm not wrong about these things, can someone explain to me what's going on in an aging battery?
--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    vtmaps wrote: »
    .... can someone explain to me what's going on in an aging battery? --vtMaps

    Well, you pretty much have it as I also understand it. Mostly, things are "falling apart" in an older battery, and as it gets worse, their efficiency goes downhill, gassing increases, and so on. Much like older humans :cool:

    Check out the BIG battery faq http://www.batteryfaq.org/
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  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    vtmaps wrote: »
    I've been thinking again, so I am confused. I have read here and elsewhere that as a battery ages its internal resistance increases. I have also read that as a battery ages its absorb end amps increases. This doesn't make sense to me. At a given absorb voltage I would expect (ohms law) that as internal resistance increases (with age of battery) the current (end amps) through that battery should decrease.

    Am I wrong about end amps increasing in an aged battery? Am I wrong about internal resistance increasing in an aged battery? Am I wrong about ohms law? If I'm not wrong about these things, can someone explain to me what's going on in an aging battery?
    --vtMaps

    Batteries are not a electronic component and don't obey ohms law like a resistor would.

    In a new battery the fresh uniform surface of the plate is easily converted to and from sponge lead during the charge/discharge cycle. As the battery ages (but is still good) less of the plate surface is available for ion flow for the chemical conversion process due to wear, cracks, sulfation and needs more current to cause deeper reactions on the still good surfaces of the plates to complete the chemical reaction for full cell voltage.
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    mike90045 wrote: »
    Mostly, things are "falling apart" and as it gets worse, their efficiency goes downhill, (and) gassing increases. Much like older humans

    So this is what I have to look forward to as I get older? Thanks a lot Mike! :cry:
    Hahahaha

    .
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    nsaspook wrote: »
    Batteries are not a electronic component and don't obey ohms law like a resistor would.
    Fair enough, but surely they can be viewed as a 'black box' built from components. I would imagine that inside that black box would be a voltage source in series with a resister (representing the internal resistance), and also another resister in parallel with the voltage source which causes the self discharge of the battery
    nsaspook wrote: »
    As the battery ages (but is still good) less of the plate surface is available for ion flow for the chemical conversion process due to wear, cracks, sulfation and needs more current
    I understand that it needs more current, but why does more current flow through it (given that it has a higher internal resistance)?
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    vtmaps wrote: »
    Fair enough, but surely they can be viewed as a 'black box' built from components. I would imagine that inside that black box would be a voltage source in series with a resister (representing the internal resistance), and also another resister in parallel with the voltage source which causes the self discharge of the battery

    I understand that it needs more current, but why does more current flow through it (given that it has a higher internal resistance)?
    --vtMaps

    There are complex model of batteries as electronic devices but because it's a chemical process based on ion charge flow (in both directions at the same time inside the battery) and is very dependant on external factors like temperature, acid concentration, etc ... instead of simple electron based charge movement in wires or devices it's actually very hard to plug electronic paramaters into a black-box and have it work correctly.
    http://doc.utwente.nl/64556/1/BatteryRep4.pdf
    The absorption voltage is a fixed voltage that the charge controller sets and monitors the current flow into the battery as a indicator of how charged the battery is. In an aged battery the internal voltage (at the same SOC compared to a new battery) of the cell rises slower and when the lead/acid chemical reaction effectlvely stops at a set charging voltage the difference between the chargers voltage and the internal cell voltage is larger.

    The difference in voltage might be small but the internal resistance even in a poor cell can still be in the milliohm range. At this point you can use ohms (really KVL) law to see that at a given resistance more voltage (charger volts - internal cell volts ) will cause more current to flow until the internal resistance is fairly high.

    http://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws#Kirchhoff.27s_voltage_law_.28KVL.29
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps

    We need to look at the entire system. We got a charger, cable, and a battery. All chargers are current limited voltage sources. A simple equivalent circuit of the battery consists of a resistor and capacitor in series.

    Charging current is defined by:

    I_charge = (V_charger - V_bat) / (R_cable + R_bat)

    As battery resistance increases, charge current decreases. It will take longer to fully charge the battery.

    I suggest you recheck your source.

    Cal
    vtmaps wrote: »
    I understand that it needs more current, but why does more current flow through it (given that it has a higher internal resistance)?
    --vtMaps
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps

    What you stated is generally true but it also depends on how the battery has been treated during its lifetime and its progressive failure mode.

    One situation that would work the opposite is a battery that has be subjected to continuously over-voltage float condition. This causes accelerated oxidation on the positive plate grid resulting in a dramatic increase in series resistance. It will not deliever much of its capacity before the inverter quits due to undervoltage and when recharge is attempted it will rise in terminal voltage quickly, reaching absorb voltage limit, with much lower then original/younger absorb current.
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    calbiker wrote: »
    We need to look at the entire system. We got a charger, cable, and a battery. All chargers are current limited voltage sources. A simple equivalent circuit of the battery consists of a resistor and capacitor in series.

    Charging current is defined by:

    I_charge = (V_charger - V_bat) / (R_cable + R_bat)

    As battery resistance increases, charge current decreases. It will take longer to fully charge the battery.

    I suggest you recheck your source.

    Cal

    The simplest close model for a battery charge/discharge cycle is the two well model of available charge and bound charge like KiBaM. The "resistor and capacitor" circuit model is totally inaccurate for any use in a true energy monitoring system as the energy conversion from electrical to chemical and back is not based on simple electrical circuit theory.

    I use a very simple version of this in my multi-battery monitor to schedule battery recharge selection.
    Another example are the electrical circuit models used in electrical
    enginering [7], which focus on the electrical properties of the battery. Although
    these models describe the battery accurately, they are not suitable to be used
    in the setting of the performance models because of the detailed description,
    which would make the combined model unmanageable. What is needed, is an
    abstract model which focuses on the important battery properties and their
    effects only. Two analytical models are good candidates: the Kinetic Battery
    Model (KiBaM) by Manwell and McGowan [8, 9, 10] and the diffusion based
    model by Rakhmatov and Vrudhula [11]. These two models describe the battery
    using only two differential equations.

    http://doc.utwente.nl/64866/1/battery-model.pdf
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps

    I wasn't interested in modeling the battery. I was explaining the effects of increased battery resistance on charging. For that, the equivalent circuit of resistor and capacitor is perfect. Do you have any problems with this relationship?

    I_charge = (V_charger - V_bat) / (R_cable + R_bat)

    Cal
    nsaspook wrote: »
    The simplest close model for a battery charge/discharge cycle is the two well model of available charge and bound charge like KiBaM. The "resistor and capacitor" circuit model is totally inaccurate for any use in a true energy monitoring system as the energy conversion from electrical to chemical and back is not based on simple electrical circuit theory.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps

    cal,

    Did not want to argue with your R & C post. BUT, to me, there is nothing simple about the behavior of battery charging. There are many things going on during the process. The nature of the charge process and apparent resistance, and thus current flow depends a lot upon the nature of the discharge. "Simple" notions of Resistance and Capacitance cannot take this and of the many other variables regarding battery charging cannot take this into account.

    I am not claiming to know a lot of this. But the more that I look at it, the less simple that it appears to be. nsaspook has been doing work on this from the perspective of designing and implementing a Battery Monitor, and is one of the folks I listen to. MHOs, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps

    Don't let the capacitor throw you off. Replace it with a pure voltage source if you want to. All I did was use Kirchoff's voltage law to derive a current equation. This current is valid for only a moment in time. Yes, battery voltage (and resistance) are very complex. That doesn't matter. At any given point in time during the charge sequence, we can derive a specific battery voltage and resistance. It's just Ohm's Law.

    Let's look at it another way. The charging voltage at the battery terminals is:

    V_bat_T = V_bat + (I_charge * R_bat)

    Likewise the discharging voltage at the terminals is:

    V_bat_T = V_bat - (I_discharge * R_bat)

    You've probably seen those equations before. The discharge battery resistance is not equal to the charge battery resistance. They are easy to measure though. I do it all the time.

    Cal
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps

    Some are interested in the why of things. Having a simple answer is teriffic. But, to me, it is of no particular value. Yes the charge current at any instant is exactly what it is, as is the resistance. But, I'll let the charging source deal with that. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps

    The purpose was to answer the opening question. And it did that.

    Cal
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    calbiker wrote: »
    I wasn't interested in modeling the battery. I was explaining the effects of increased battery resistance on charging. For that, the equivalent circuit of resistor and capacitor is perfect. Do you have any problems with this relationship?

    I_charge = (V_charger - V_bat) / (R_cable + R_bat)

    Cal

    That only shows the static value of current flow but is meanless to understand the actual energy being stored in the battery and the current state of charge without a table of possible values that have been derived from a complete constant discharge/recharge cycle of a battery in the same condition as the original data. As soon as loads and charging currents are not constant, a linear model of a battery just using voltages, resistance , capacitors and current flows becomes wildly inaccurate as people with simple battery monitors know. What we see as the internal resistance load and the current flow into a battery at the upper end of charge is mostly converted to exothermic heat and is wasted due to the required chemical energy needed to drive the last ions into the battery electrodes. The absorption voltage/end amps of a battery as it ages is a compromise on battery life and possible energy storage capacity. As a battery ages the surface area for easy reactions decreases so it takes more energy/current to achieve the same energy storage in the deeper structure of the plates and sooner or later the point arrives where the heat and stress from that extra energy destroys it or caused permanent loss of capacity.
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    nsaspook wrote: »
    As soon as loads and charging currents are not constant, a linear model of a battery just using voltages, resistance , capacitors and current flows becomes wildly inaccurate as people with simple battery monitors know.

    That's not right. The equation does not become wildly inaccurate. It is always 100% accurate. It seems to me you're asking for the holy grail of battery dynamics/modeling, while I'm showing why charging current decreases with increasing battery resistance. OK?

    Cal
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Battery aging, internal resistance, and end amps

    Interestingly, it is a bit more complex... Depending on the charging cycle, you have current mode (bulk) and voltage mode (absorb, float).

    During Bulk mode, resistance of wiring/battery/etc. does not affect the charging current as the charging source is simply outputting its maximum/programed current--Up until the Absorb Voltage set point is reached. At that point "excessive resistance" (wiring, battery) does affect the current flow as the charge controller has now entered a voltage controlled mode--And excessive resistance (too long of cable, too small of cable) will "trick" the charger into entering "absorb" mode sooner (battery appears to be at a higher state of charge than it really is)--and probably stretch out the charging time (and if solar--losing charging energy during the middle of the day as the charge controller throttles back).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    calbiker wrote: »
    That's not right. The equation does not become wildly inaccurate. It is always 100% accurate. It seems to me you're asking for the holy grail of battery dynamics/modeling, while I'm showing why charging current decreases with increasing battery resistance. OK?

    Cal

    Ohms law is 100% correct but it only explains the visible effect not the cause of the charging current changes with increasing battery resistance during aging. Not understanding the source of the resistance change can be misleading to a person with poor battery performance and could stop them them from using simple countermeasures to possibly restore most of the lost capacity of old batteries. I have no problem with your answer but I was also answering another question "can someone explain to me what's going on in an aging battery" with a segue into electrochemical models explaining behaviour normal electronic circuit theory can't.
    Am I wrong about end amps increasing in an aged battery? Am I wrong about internal resistance increasing in an aged battery? Am I wrong about ohms law? If I'm not wrong about these things, can someone explain to me what's going on in an aging battery?

    http://forum.solar-electric.com/showthread.php?14493-Battery-aging-internal-resistance-and-end-amps&p=108387#post108387

    PS: I've already got the "holy grail" of battery models http://www.nsnam.org/doxygen/classns3_1_1_rv_battery_model.html , making it fit into a cheap 8 bit micro-controller with 4K of sram memory is the problem but there's not a lot else to do during the long wet Oregon winter, spring, most of summer and some of fall. :cry:
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    RCinFLA wrote: »
    What you stated is generally true but it also depends on how the battery has been treated during its lifetime and its progressive failure mode.
    One situation that would work the opposite is a battery that has be subjected to continuously over-voltage float condition. This causes accelerated oxidation on the positive plate grid resulting in a dramatic increase in series resistance. It will not deliever much of its capacity before the inverter quits due to undervoltage and when recharge is attempted it will rise in terminal voltage quickly, reaching absorb voltage limit, with much lower then original/younger absorb current.
    Interesting... let's see if I understand this correctly...

    --In a normally aged battery internal resistance increases and end amps increases.

    --In a battery that has aged because of positive grid corrosion internal resistance increases and end amps decreases.

    This leads to the next question: What are the characteristics (resistance and end amps) of an aged sulfated battery?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps

    VT, IIRC you can get both Pos plate corrosion AND Sulphation
     
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  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    BB. wrote: »
    Interestingly, it is a bit more complex... Depending on the charging cycle, you have current mode (bulk) and voltage mode (absorb, float).

    -Bill

    During constant current charging, this equation is still valid.

    I_charge = (V_charger - V_bat) / (R_cable + R_bat)

    But it makes more sense to solve for V_charger:

    V_charger = I_charge * (R_cable + R_bat) + V_bat

    This relationship is valid as long as V_charger is less than V_charger_max.

    As you mentioned, resistance has a big impact on how long the charger stays in constant current. As R_bat increases, the system may never get into constant current mode. Charging will be very slow.

    Cal
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    nsaspook wrote: »
    PS: I've already got the "holy grail" of battery models http://www.nsnam.org/doxygen/classns3_1_1_rv_battery_model.html , making it fit into a cheap 8 bit micro-controller with 4K of sram memory is the problem but there's not a lot else to do during the long wet Oregon winter, spring, most of summer and some of fall. :cry:

    I just spent a few minutes reviewing this link. It appears the program is doing something similar to what I've done. They measure alpha & beta during discharge. They probably get battery resistance or Peukert constant (which is very similar to resistance) from the two measurements. After that, it's a matter of continually measuring voltage.

    Cal
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    calbiker wrote: »
    I just spent a few minutes reviewing this link. It appears the program is doing something similar to what I've done. They measure alpha & beta during discharge. They probably get battery resistance or Peukert constant (which is very similar to resistance) from the two measurements. After that, it's a matter of continually measuring voltage.

    Cal

    Not really, the Rakhmatov Vrudhula non-linear battery model is explained here.
    http://doc.utwente.nl/64556/1/BatteryRep4.pdf section 5 5.2 5.3

    Your ideas are likely to be from the electrical model from section 4 or 5.1 Peukert’s law.

    My system is (still being modified for the uC controller) using a simplified version of the Stochastic modified KiBaM combined with the electrical and Peukert’s law models as a counter-check monitors .
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    nsaspook wrote: »
    Not really, the Rakhmatov Vrudhula non-linear battery model is explained here.
    http://doc.utwente.nl/64556/1/BatteryRep4.pdf section 5 5.2 5.3

    Your ideas are likely to be from the electrical model from section 4 or 5.1 Peukert’s law.

    I'm not convinced that's the holy grail of battery modeling. The program is concerned with estimating battery life. It calculates life from two discharge measurements: alpha and beta. The greatest factor affecting battery life is depth of discharge. Also, time duration in discharged state, charge rates, temperature, and battery conditioning are major factors in life calculations. These factors don't appear in the model. And I'm not sure what relevance those models have in this discussion. Oh yes, there is one model discussed in you link that is relevant. That's the electrical model consisting of a resistor and capacitor. I don't know anything about battery diffusion process, but resistors are very tangible. ;-)
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    calbiker wrote: »
    That's the electrical model consisting of a resistor and capacitor. I don't know anything about battery diffusion process, but resistors are very tangible. ;-)

    As you can also see it's the worst at actually matching what a battery does even with a huge about of parameters and tables, the battery electrochemical process is pretty simple and once understood will make the battery diffusion (what really happens, not resistors, capacitors and voltage sources) process tangible. :D

    The "life of the battery" in the models is the energy life during discharge not the overall life of a battery
    The total life of a battery is affected by all those factors and they are included in my programs battery wear/life model. The "holy grail" thing is a joke but NS3 does provide a good foundation for building a better mouse trap.

    As for "what relevance those models have in this discussion" that's up to the viewer.
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    nsaspook wrote: »
    As for "what relevance those models have in this discussion" that's up to the viewer.

    So the viewer decides between the electrical model where battery resistance is easily derived from a few measurements vs. the Rakhmatov Vrudhula model. In this model the "viewer" also takes a few measurements. Then he plugs those measurements in some differential equations & Laplace transforms. The final result is a measure of battery capacity. There are no units of resistance anywhere in the calculation. Now that's really odd, as resistance is the parameter we're looking for!

    BTW, what processor are you using of those calculations? I've been playing around with a Freescale MCF5223 microcomputer.
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    calbiker wrote: »
    So the viewer decides between the electrical model where battery resistance is easily derived from a few measurements vs. the Rakhmatov Vrudhula model. In this model the "viewer" also takes a few measurements. Then he plugs those measurements in some differential equations & Laplace transforms. The final result is a measure of battery capacity. There are no units of resistance anywhere in the calculation. Now that's really odd, as resistance is the parameter we're looking for!

    BTW, what processor are you using of those calculations? I've been playing around with a Freescale MCF5223 microcomputer.

    If you look at a battery as a electrical device the resistance is relevent but as a device that stores and delivers charge via a chemical process internally it's not. The internal "resistance" is really proxy for the ease of movement of ions in a normal battery not classical electron charge carriers we see in wires and common electrical components. A battery does not obey Maxwell equations or ohms law because it does not use EM fields like a generator or even a PV cell. At the wire connection point to the battery then all the normal electrical equations come into play. I think the key to really understand battery operation is to look at it for what it is, not a sometimes close electrical simulation. The ion charge equations do look strange to a person who has a very good electrical background but little background in complex materials science such a semiconductors where the ideas of diffusion, doping and energy flow due to negative and positive ion flow are common.

    The basic energy control processor is a PIC18F8722 with a PIC32MX795F512L as the web and network frontend. It's just a hobby, not my "real" job. Freescale makes good stuff but it's a competitor in the business. :blush:

    http://forum.solar-electric.com/showthread.php?7396-DIY-battery-monitor-charger&p=52721#post52721
  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    Re: Battery aging, internal resistance, and end amps
    nsaspook wrote: »
    A battery does not obey Maxwell equations or ohms law because it does not use EM fields like a generator or even a PV cell.


    The battery industry will disagree with you. Battery resistance testing is well developed. I check my battery resistance every few years. It is repeatable and accurate.

    This is a good read.
    http://www.alber.com/Docs/PredictBatt.pdf

    http://www.buchmann.ca/article25-page1.asp
    http://www.buchmann.ca/Chap9-page2.asp
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery aging, internal resistance, and end amps
    calbiker wrote: »
    The battery industry will disagree with you. Battery resistance testing is well developed. I check my battery resistance every few years. It is repeatable and accurate.

    I don't disagree that it's a good quick check as the whole point is to provide electrical power to a load.

    Repeatable and accurate is great until something changes. If you had a two identical sealed generators with only a fuel port, one run by a sugar water powered hamster on a wheel and another generator powered by hot candle wax. Both generators are producing less and less power and your job is to get them both running at full power again by adding fuel. All your external measurements would be useless unless you know what's really inside. A little hot wax on the hamster might even make more power for a short time.