In a magical system similar to that of Eragon, where magic depletes the body as if they had manually done the same action as the spell effect (e.g. Magically moving a boulder would deplete the body of energy as if the caster had manually moved it, only on a much shorter time frame), how would the below "loophole" be more or less balanced. Just like in Eragon, a spell is committed once cast and cannot be aborted mid way if a miscalculation has been made.

I've made it possible for the caster to take energy from other living things; plants, animals and even other humans (providing there is consent) which has lead to the "logical development" of powerful casters and/or kingdoms keeping essentially "human batteries". These individuals, in return for being available to power spells, and thus reducing the risk to the caster and/or allowing them to cast more powerful spells, are given a life of absolute luxury. They're fed a high fat diet causing them to be rather obese, which is beneficial as more fat = more energy.

The obvious risk to the "battery" is the caster could kill them either intentionally or accidentally if they cast a spell with more energy required than provided, as the caster can "allocate" where the energy comes from, and will obviously state the casters energy as the least amount possible. In order limit the likelihood of the "battery" denying the caster, their contract is on pain of death so it's potential vs definite death.

With regards to "allocation" the caster must always spend some of their own personal energy as a kind of "stake". Consider the following example: A caster with 3 batteries wishes to hurriedly raise the drawbridge so he allocates 30% of energy to each battery and the remainder to himself.

The casters prefer humans, rather than animals as the "batteries" are often magic users themselves and thus can act in that capacity to defend themselves or even offensively.

It's worth noting that it's also energy available to the caster so a fit person would be able to move the boulder in the example easier than an unfit person, thus requiring less "energy". This also works based on understanding, so a person that understands metallurgy would be able to heat the enemies armour more efficiently than someone who does not.

When considering an answer I'm not sticking to "science based", but more "fridge logic", it should make sense as presented, ideally hold up to a small to medium amount of scrutiny, but I'm not expecting this system to hold up once maths is truly applied.

Small update

I don't want to nerf this concept to the point of being unviable, I just want to stop it getting out of hand or becoming the norm, despite the moral quandaries.

Answer awarded on

In order of descending priority:

  1. A logical means of preventing large battery farms, I do want to stay away from "only 3 batteries can be linked for plot reasons"
  2. A logical reason as to why "I allocate 1 joule of energy to myself", my thoughts are this should mostly be done in percentages as the cost is "unknown"
  3. How can the ethical mage/protagonist achieve the same ends without the dubious morality. i.e. a human can only become so fit
  4. Logical consistency with a "common understanding"
  • 1
    $\begingroup$ "becoming the norm, despite the moral quandaries" - unless this is permanently harmful to the volunteer, this is less ethically questionable than conventional work. A stonemason-wizard (say) could hire workers to shift the stone mundanely, expending the same energy as it takes to magically shift. But magic doesn't have to spend time placing rollers and risking injuries - instead, workers sit safely in a nearby room and let the mason use their strength. $\endgroup$
    – Anon
    Commented May 28, 2021 at 5:21

5 Answers 5


Transfer Costs

A caster using power from someone else acts like a heat pump. For every unit of energy they take they must expend some energy to make the transfer. You can then set the ideal optimal ratio for your setting. If a caster must expend 1 joule to transfer 4 joules, then it doesn't make sense for him to use more than 4 human or human equivalent power sources at any given time. These power transfers could also be a skill that takes practice to develop, or a "muscle" that must be strengthened, with the most powerful casters being able to transfer energy more efficiently than weaker ones.


Transferring power through a person could also have harmful effects on the caster as the amount of power increases. One person-equivalent level of power is perfectly natural and harmless, but as the amount of power increases you start doing tissue damage. For electrical current, heat generated by resistance increases with the square of the current. If magical resistance were modeled in a similar way, the harm to the caster would increase exponentially and make large scale energy harvesting lethally dangerous.

You could establish both acute and chronic effects for this sort of overuse. Perhaps the more power you draw, the less time you can sustain the transfer for before sustaining tissue damage, which sets a limit to the energy a caster can access over a certain amount of time, regardless of how much is available. You could also have long term "scarring" for repeated or extreme overuse of power, which reduces the caster's ability to draw power in the future.

  • $\begingroup$ I like both of these ideas, do you think they could work in conjunction or would that just make it so there's no "pro's" to this form of transfer. I still want to allow this and have it viable. $\endgroup$
    – Joshua
    Commented May 28, 2021 at 0:23
  • 2
    $\begingroup$ You could absolutely use them in conjunction, and you could "tune" the settings of your world to allow the kind of system dynamics you're looking for. Each one sets a different sort of ceiling on magic use. The transfer costs set an absolute cap to the amount of magical energy a caster can transfer at a time (peak power), while resistance sets a soft cap on the continuous throughput of a caster (max continuous power). However, you would need to run the math to make sure one didn't render the other totally irrelevant. $\endgroup$ Commented May 28, 2021 at 0:55
  • $\begingroup$ Another limit that rewards work is finding volunteers. A hospital could keep some paid energy-source staff to help their healers work, but this is cultural - someone literally draining your vitality isn't fun, but it could be lucrative. $\endgroup$
    – Anon
    Commented May 28, 2021 at 5:18


Like many magic systems, similar things work better together. An "Evil" mage can enforce this with blood-magic and mind-control (drugs, etc). This is scalable, and works especially well for a hive-mind villain. It also explains why minions are nearly identical clones. However, mind-control isn't perfect.

"Good" mages can attune to their apprentices/familiars, and if they need a miracle to save Atlantis (say) then having an entire city assisting (at low efficiency) with the effect.

This also means that having an engineer, an artist, and an expert as part of the casting-group will allow increased efficiency.

Battery farms: Attunement to many minions is hard, and confines the caster's options and even thoughts.

Caster participation: If all the minions are missing their left hand, the caster's left hand takes the full relevant cost. Mentally, it's similar - any part of yourself that you are unique in having, you can't pass off that part of the cost.

Ethical mages: "My apprentice knows me, and has volunteered to share the cost (maybe as part of their training)." This can become a pyramid scheme for ancient wizards.

Logical consistency: It's your world to decide whether this will fit in.

  • $\begingroup$ I like this affinity idea, and the picture of Wizard MLMs cracks me up, the Affinity could be a way to reduce the "resistance" in the previous answer. I also really like your participation part too. $\endgroup$
    – Joshua
    Commented May 28, 2021 at 2:45
  • $\begingroup$ I envisioned that part to make sure an Evil caster can't fob the full cost off on minions; but it has some mice consequences in enforcing schools of magic too. I'm not familiar with the term "MLM", though? (Experience tells me not to search unknown acronyms.) $\endgroup$
    – Anon
    Commented May 28, 2021 at 5:13
  • $\begingroup$ MLM = Multi Level Marketing $\endgroup$
    – Joshua
    Commented May 28, 2021 at 9:05

Batteries: connecting them in Series vs connecting them in Parallel

Referencing your questions I think the following might help. If you are using people as magical batteries then apply a set of magical laws that are similar to those applying to real world batteries. Note; The issue of resistance mentioned by other posters takes care of question 2.

Series connections involve connecting 2 or more batteries together to increase the voltage of the battery system, but keeps the same amp-hour rating. Keep in mind in series connections each battery needs to have the same voltage and capacity rating, or you can end up damaging the battery.

In this context the flow of magic from the 'battery farm' flows through each member of the farm in turn on its way to the user.

Advantage - this gives your magician large amounts of power to play with so powerful spells can be performed.

Disadvantage - significant differences in the health/fitness of different members will seriously strain the circuit to the point where if the strain becomes to much the magician risks the magical equivalent of a battery short circuit or fire. If/when this happens individual members of the farm will 'short' out (from weakest to strongest) in series. The risk and seriousness of the damage inflicted also increases exponentially over time. This means that while yes, a magic user can potentially draw huge amounts of power/create powerful spells with this type of linkage you also quickly (within seconds?) reach the point where the weakest members of the farm will suffer traumatic damage/die and the circuit will short/collapse.

When this happens other members of the farm will also suffer some degree of damage as the effect of the short passes through them. Importantly since by default the magician will still be 'linked' to the battery if/when it shorts he or she also risks suffering traumatic, even potentially fatal damage as well. (You can play with all the potential variables to fit your idea and get a % risk factor.) P.S. Large farms used like this would also be terribly expensive to maintain because all members have to be kept at the same level (more or less) of health all the time. One bad cold could ruin your day.

So just like real world batteries differences in the health/fitness of individual members risks disruption to the the circuit. This puts strict limits on the total size of the farms anyone can have because if you want a 100 person battery they all need to have more or less the exact same level of fitness (say within 1 or 2%) and obviously the stronger and fitter they are the better.

Parallel connections involve connecting 2 or more batteries together to increase the amp-hour capacity of the battery bank, but your voltage stays the same. A parallel connection is not meant to allow your batteries to power anything above its standard voltage output, but rather increase the duration for which it could power equipment.

This series is much safer and lets magicians perform lower level spells for very long periods of time. Members of the farm can have varied levels of fitness without risk. So if when drained a weaker member of the farm reaches a critical health point they can simply 'drop out' of the circuit without disrupting it. The only effect being that the amount of time left in which to perform your spells drops to. Nobody, including the mage dies and anyone can be part of the battery because the level of fitness isn't that important (within reason).

So you end up with 2 models - (A) short and very powerful where spells have potentially life threatening consequences for everyone involved or (B) long term, lower level spells that are more or less risk free for all concerned. Chose the former at your peril.

One last point concerning the 'farms'. The obvious limitation on large farms would simply be the logistics of taking large groups of people with you wherever you go. Large farms would therefore limit your mobility restricting you to a fixed point. Particularly if one of the limitations is the distance between the magician and his/her 'farm'.

Presumable there doesn't have to be a fixed, physical connection between them. But if the amount of power that can be drawn from a farm also drops fairly steadily with distance a magician may well be limited as to how far away from his farm he/she can be when performing spells and of course the stronger the spell the closer they have to be. This is another element of the resistance problem i.e it leads to a drop in power over distance as in any electrical circuit.

  • $\begingroup$ Good concept, but it needs work. As I understand it, chemical batteries which are (relative to the string) understrength will heat up more, which increases resistance and creates a positive feedback loop. Intelligent systems can shut down that string (or even splice out that cell at the cost of output voltage). There's also the potential to use multiple strings for a middling effect. $\endgroup$
    – Anon
    Commented Jun 2, 2021 at 1:19
  • $\begingroup$ Technically yes but those kind of systems have external (independent) monitoring systems. In this case the magician is part of the circuit - they have no choice. I suppose you could use an apprentice or accomplice to 'monitor the state of your human batteries for you but I don't see how the magician him or herself could do so. Not when they already have to concentrate on (a) channeling/stabilizing all the power that's flowing into them and (b) forming that power into a coherent spell - and doing both simultaneously! $\endgroup$
    – Mon
    Commented Jun 2, 2021 at 5:43
  • $\begingroup$ I was suggesting that the (presumably intelligent) people in the circuit might be self-monitoring and self-adapting. The mage could have this occur automatically - but willing helpers are likely to hold on for longer than minions would, for a worthy cause. $\endgroup$
    – Anon
    Commented Jun 2, 2021 at 5:46
  • $\begingroup$ That assumes that the living things or people being used as batteries are perhaps active participants in the process. The OP makes no mention of this BTW and seems to hint that this is not in fact the case. To be an active part of the process (as opposed to just feeling the physical effects) they would probably need to be at least sensitive to magic, if not actually strong enough to to practice it themselves. Anyway I think the implication is that any human being can be part of a battery and that, as in the case of most of these kind of settings magical abilities are uncommon if not rare. $\endgroup$
    – Mon
    Commented Jun 2, 2021 at 5:56
  • $\begingroup$ Also if people had to be active participants - i.e. be able to proactively sense the status of the magical process as it happens then you would be further limiting the number of potential subjects. People who had no magical 'senses; could not be used as batteries at all. P.S. I can see how the mage might 'meld' with the battery and be able to sense them in a collective way, perhaps even feel which ones were failing/weakening faster. I just don't see how he focus on 'managing' the individual parts of his 'farm' and cast at the same time to. Especially with large numbers of batteries. $\endgroup$
    – Mon
    Commented Jun 2, 2021 at 6:06

Limiting battery farms

I like and second Shawn's answer for how to limit large battery farms and fully support it.

As one possible suggestion along the same lines instead of (or possibly in addition to) putting a hard limit via resistance and energy use as per shawn's answer you could put more of a soft limit via diminishing returns. The more energy a user transfers the more energy is 'lost' to waste heat (or some other form of entropy). Thus as more humans are added you get diminishing returns. In theory you could have 100 people all supporting one user, but you'll only get slightly more energy as you would get from 4 people supporting you, while exhausting all 100, so it just doesn't make sense to have much more then 3-4 people supporting a caster at once.

As a side note this still allows a caster to rotate his support casters, 'using up' 4 supports energy then sending them back inside and calling in the next 4 to support his second spell.

Humans only

You mentioned the fridge logic of asking why people didn't just use animals as energy farms. Your idea is interesting, but I don't think that stops animals from being used. You probably don't want your human-batteries using magic to defend themselves since that is wasting energy that your lead-caster could likely use more effectively (or worse, using more energy then he knew was expanding resulting in his accidentally killing you because he misjudged how much remaining energy you had available to support a spell). It is generally more efficient to have one person in charge of magicking. Besides obese unfit humans probably aren't much better at defending themselves then a fit horse, and the horse has more available energy still!

If you're threatening to kill people if they don't participate in your potentially lethal magic system you may also want to not teach them how to defend themselves via magic...

Thus as a second limit I'd recommend saying that energy transfer is more efficient if the human is consciously supporting it, a skill that mere animals can't be properly trained in. So basically humans make a far more efficient battery source then animals do. You would probably want to say that having a human supporting you instead of an animal raises the effective limits caused by resistance/energy transfer/diminishing returns as per shawn's suggestion.

This could easily be tied into the part about needing a humans consent before you can drain power from them, just as a human can support you in transferring energy more efficiently they could also resist you, making energy transfer too inefficient to be useful.

I'd also suggest making it clear that the limit on energy transfer is about the effective energy transferred, not how many people that energy comes from. this eliminates the potential loophole of brining 4 elephants to the battlefield because 4 of them possess massively more raw energy then 4 humans. All that raw energy of the elephants doesn't help if the caster can only use a fraction of it, and the humans could have increased the amount of total energy the caster could handle.

Ethical casting

Now your last question of address your question on how an 'ethical' mage can still use other's humans as battery farms. Have him allocate a larger (adjusted) energy cost to himself then any of the others available to him. Then say that if the lead caster dies first that interrupts the spell (since he can no longer act as an energy conduit for the other users energy). Thus he works sort of like how a surge protector works in an electrical system. He ensures he dies first if anyone would die, and his death in turn saves the lives of all his support casters.

You may want to allow support casters to 'sense' how much of their energy, as a percentage of total energy cast, is going to be expanded before a spell is cast. That way a support caster can have an idea rather the primary caster is going the ethical route of acting as surge protector to protect his support casters or if the primary caster is putting his supports at risk.

As an intermediate option someone may want to try having a primary caster holding some poor victim (let's call him Vic) hand. Then Vic in turn is in contact with all the other support casters. In this case the main caster could potentially make Vic the surge protector, burning Vic out to save the other casters, rather then putting his own life on the line by acting as surge protector.

Of course you may not like having a Vic as a surge protector. If so it's easy to prevent by saying if Vic burns out the caster has to provide all the energy that was originally coming from 4 humans, thus almost guaranteeing the caster also dies if Vic dies. Thus you have the flexibility of choosing rather you want to have a Vic middle man or not as a possibility in your system.

No animals were harmed with this casting?

As another alternative for an 'ethical caster' you could have your ethical caster utilize animals as an energy source instead of humans. If you stuck with my 'humans only' advice above that would mean lowering the effective throughput the caster could manager for his spells, relative to someone supported by humans. However, it may still allow him somewhat close to the same energy use without the risk of killing humans. Having a protagonist have to struggle to overcome a power disadvantage due to his unwillingness to risk other's is a common staple of plenty of fiction, so that isn't necessarily a problem so long as the hero has enough power to have some chance of winning.

Of course you could argue rather that counts as 'ethical' depends on how much one values the life of an animal relative to that of a human. If you don't want your hero to be killing helpless animals you could instead ensure that he only uses larger animals. If the animal has a large enough energy reserve, and a low enough efficiency on how much energy they can transfer, it's entirely possible that the caster would risk killing himself before he was able to draw enough energy from his animal to put the animal at risk of death.

Plus, I just love the idea of the enemy being shocked when your hero suddenly bashes down their gates by riding a giant, armored, war elephant and starts flinging around spells nearly as strong as what your human supported casters can manage while being mobile and, you know, riding a @)#(%#^ elephant!! The only thing more awesome that that would be having your wizard riding in on, say, a necromantically raised T-rex!

enter image description here


Mana shaping

There is some kind of mental coordination required to channel the energy into/through the spell being cast.

Channelling your own energy is trivially easy, channelling somebody else's alongside your own is hard, but a skill that can be learned and practiced. Channelling 100 people at the same time is virtually impossible, especially if you have to control it well enough not to kill yourself or any of the people in the process.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .